Data storage system with multiple durability levels

ABSTRACT

A data storage system includes multiple head nodes and multiple data storage sleds mounted in a rack. For a particular volume or volume partition one of the head nodes is designated as a primary head node for the volume or volume partition. The primary head node is configured to store data for the volume in a data storage of the primary head node and cause the data to be replicated to a secondary head node. The primary head node is also configured to cause the data for the volume to be stored in a plurality of respective mass storage devices each in different ones of the plurality of data storage sleds of the data storage system.

BACKGROUND

The recent revolution in technologies for dynamically sharingvirtualizations of hardware resources, software, and information storageacross networks has increased the reliability, scalability, and costefficiency of computing. More specifically, the ability to provide ondemand virtual computing resources and storage through the advent ofvirtualization has enabled consumers of processing resources and storageto flexibly structure their computing and storage costs in response toimmediately perceived computing and storage needs. Virtualization allowscustomers to purchase processor cycles and storage at the time ofdemand, rather than buying or leasing fixed hardware in provisioningcycles that are dictated by the delays and costs of manufacture anddeployment of hardware. Rather than depending on the accuracy ofpredictions of future demand to determine the availability of computingand storage, users are able to purchase the use of computing and storageresources on a relatively instantaneous as-needed basis.

Virtualized computing environments are frequently supported byblock-based storage. Such block-based storage provides a storage systemthat is able to interact with various computing virtualizations througha series of standardized storage calls that render the block-basedstorage functionally agnostic to the structural and functional detailsof the volumes that it supports and the operating systems executing onthe virtualizations to which it provides storage availability.

Some block-based storage systems utilize a server node and multiplestorage nodes that are serviced by the server node or dual server nodesthat service multiple storage nodes. For example, a storage area network(SAN) may include such an architecture. However, in such systems, afailure of one or more of the server nodes may result in a large amountof storage capacity served by the server node(s) being rendered unusableor may result in significant decreases in the ability of the storagesystem to service read and write requests.

In order to increase durability of data, some block-based storagesystems may store data across multiple devices in multiple locations.For example, a SAN may span multiple locations such as differentfacilities or different geographic locations. Such systems may utilize acommon control plane to manage data in the multiple locations. However,in such systems, a failure of a component of the common control planemay impact a large quantity of storage capacity and render the largequantity of storage capacity unavailable. Also, such systems may requireextensive networks to move data between the multiple locations and mayalso result in high latencies for data recovery due to data beinglocated across the multiple locations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a data storage unit comprising head nodes and datastorage sleds, according to some embodiments.

FIG. 2 is a block diagram illustrating a provider network implementingmultiple network-based services including a block-based storage servicethat includes data storage units, according to some embodiments.

FIG. 3 is a block diagram illustrating head nodes and data storage sledsof a data storage unit storing block storage data in response to a writerequest, according to some embodiments.

FIGS. 4A-4B are block diagrams illustrating a log storage and index of ahead node storage, according to some embodiments.

FIG. 5 illustrates a partial view of a data storage unit that storesportions of a volume partition in multiple mass storage devices inmultiple data storage sleds on multiple shelves of the data storageunit, according to some embodiments.

FIGS. 6A-B illustrate columns of mass storage devices storing differentportions of a volume partition, according to some embodiments.

FIG. 7 is a high-level flowchart illustrating operations performed by ahead node in response to a request to store data in a data storage unit,according to some embodiments.

FIG. 8A is a high-level flowchart illustrating operations performed by ahead node in response to a failed mass storage device in a data storagesled of a data storage unit, according to some embodiments.

FIG. 8B is a high-level flowchart illustrating operations performed by ahead node in response to a failed mass storage device in a data storagesled of a data storage unit, according to some embodiments.

FIG. 9A is a block diagram illustrating a process for creating a volumeinvolving a zonal control plane, a local control plane, and head nodesof a data storage system, according to some embodiments.

FIG. 9B is a block diagram illustrating head nodes of a data storageunit servicing read and write requests independent of a zonal controlplane of a data storage system, according to some embodiments.

FIG. 10A is a block diagram of a head node, according to someembodiments.

FIG. 10B is a block diagram of a data storage sled, according to someembodiments.

FIG. 11 is a high-level flowchart illustrating a process of creating avolume in a data storage system, according to some embodiments.

FIG. 12A is a high-level flowchart illustrating a local control plane ofa data storage unit providing storage recommendations to a head node ofthe data storage unit for locations to store data in data storage sledsof the data storage unit for a volume serviced by the head node,according to some embodiments.

FIG. 12B is a high-level flowchart illustrating a head node of a datastorage unit storing data in data storage sleds of the data storageunit, according to some embodiments.

FIG. 13 is a high-level flowchart illustrating head nodes of a datastorage unit performing a fail over operation in response to a failureof or loss of communication with one of the head nodes of the datastorage unit, according to some embodiments.

FIG. 14 is a block diagram illustrating performance and/or usage metricsbeing collected and accumulated in a data storage unit, according tosome embodiments.

FIG. 15 illustrates interactions between a local control plane, headnodes, and data storage sleds of a data storage unit in relation towriting data to mass storage devices of a data storage sled of a datastorage unit, according to some embodiments.

FIG. 16 is a high-level flowchart of a head node of a data storage unitflushing data stored in a storage of the head node to a data storagesled of the data storage unit, according to some embodiments.

FIG. 17 is a high-level flowchart of a sled controller of a data storagesled processing a write request, according to some embodiments.

FIGS. 18A-D illustrate a data storage unit with redundant network pathswithin the data storage unit, according to some embodiments.

FIGS. 19A-C illustrate a data storage unit configured to allow scalingof storage capacity and processing capacity, according to someembodiments.

FIG. 20 is a block diagram illustrating an example computing system,according to some embodiments.

While embodiments are described herein by way of example for severalembodiments and illustrative drawings, those skilled in the art willrecognize that the embodiments are not limited to the embodiments ordrawings described. It should be understood, that the drawings anddetailed description thereto are not intended to limit embodiments tothe particular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope as defined by the appended claims. The headings usedherein are for organizational purposes only and are not meant to be usedto limit the scope of the description or the claims. As used throughoutthis application, the word “may” is used in a permissive sense (i.e.,meaning having the potential to), rather than the mandatory sense (i.e.,meaning must). Similarly, the words “include”, “including”, and“includes” mean including, but not limited to.

DETAILED DESCRIPTION

According to one embodiment, a data storage system includes a rack, aplurality of head nodes mounted in the rack, and a plurality of datastorage sleds mounted in the rack. For a partition of a volume to bestored in the data storage system, a particular one of the head nodes isdesignated as a primary head node for the volume partition and anotherone of the head nodes is designated as a secondary head node for thevolume partition. In response to receiving a write request for thevolume partition, the head node designated as the primary head node forthe volume partition is configured to write data included with the writerequest to a storage of the head node designated as the primary headnode and cause the data included with the write request to be replicatedto the other head node designated as the secondary head node.Furthermore, the head node designated as the primary head node for thevolume partition is further configured to cause respective parts of thedata stored in the storage of the head node to be stored in a pluralityof respective mass storage devices each in different ones of theplurality of data storage sleds of the data storage system. For example,a data storage system may store data in a storage of a primary head nodeand replicate the data to a storage of a secondary head node. Then,after a certain amount of time has passed, a certain amount of data hasbeen written for the volume partition, or in response to anothertrigger, the head node may cause the data stored in the storage of thehead node to be stored in multiple mass storage devices of differentones of the data storage sleds of the data storage system. For example,data may be stored in mass storage devices of different data storagesleds of a data storage system in a RAID array and may be erasureencoded across the multiple mass storage devices. Such a system mayprovide varying latencies for accessing stored data and differentdurabilities of the stored data based on whether the data is stored instorages of the primary and secondary head nodes or stored in multiplemass storage devices of multiple data storage sleds of the data storagesystem. For example, data stored in a storage of a primary head node maybe accessed with lower latencies than data stored across multiple datastorage sleds of a data storage system. However, data stored acrossmultiple data storage sleds of a data storage system may have higherdurability than data replicated between storages of a primary andsecondary head node. Thus, a data storage system may provide lowlatencies for recently or frequently accessed data while providing highdurability for long term storage of data or for data less frequentlyaccessed. In some embodiments, durability of data stored and replicatedin head nodes may be adjusted by varying a number of head nodes thatreplicate the data. Also, durability of data stored in mass storagedevices of data storage sleds of a data storage system may be adjustedby varying a RAID scheme or data encoding procedure used to store thedata amongst other techniques to increase data durability.

According to one embodiment, a data storage system includes a head nodeof a data storage system, wherein a plurality of data storage sleds ofthe data storage system are also included in the data storage system.The head node, when acting as a primary head node of the data storagesystem for the volume partition and in response to receiving a writerequest for a volume partition, is configured to write data includedwith the write request to a storage of the head node and cause the dataincluded with the write request to be replicated to another head node ofthe data storage system designated as a secondary head node for thevolume partition. The head node is further configured to causerespective parts of the data stored in the storage of the head node tobe stored in a plurality of respective mass storage devices each indifferent ones of the plurality of data storage sleds mounted in therack of the data storage system.

According to one embodiment, a non-transitory computer readable mediumstores program instructions for implementing a head node of a datastorage system, wherein the program instructions when executed by aprocessor cause the system to, in response to receiving a write requestfor a volume partition, write data included with the write request to astorage of a head node designated as primary head node for the volumepartition and cause the data included with the write request to bereplicated to another head node of the data storage system designated asa secondary head node for the volume partition. The program instructionswhen executed by the processor further cause respective parts of thedata stored in the storage of the head node designated as the primaryhead node to be stored in a plurality of respective mass storage deviceseach in different ones of a plurality of data storage sleds mounted in arack of the data storage system.

According to one embodiment, a data storage system includes a pluralityof data storage units, for example that are each hosted on a rack, aplurality of head nodes mounted in the rack, and a plurality of datastorage sleds. The data storage system also includes one or morecomputing devices external to the plurality of data storage unitsconfigured to implement a zonal control plane for partially controllingstorage operations related to the plurality of data storage units. Inresponse to a volume creation request, the zonal control plane isconfigured to assign a particular one of the data storage units toservice a volume requested by the volume creation request. Also, foreach respective data storage unit, at least one of the head nodes of therespective data storage unit is configured to implement a local controlplane for the respective data storage unit, wherein the plurality ofhead nodes are configured to service read requests and write requestsdirected to one or more volumes stored in the respective data storageunit independent of the local control plane and the zonal control plane.For example, a data storage system may include data storage units thatare configured to service read and write requests without the read andwrite requests being routed through a local control plane or a zonalcontrol plane of the data storage system. Also, the data storage unitsof the data storage system may continue to service read and writerequests from client devices regardless of whether communication with alocal control plane or a zonal control plane of the data storage systemis available or lost.

According to one embodiment, a data storage system includes a datastorage unit comprising a plurality of head nodes and a plurality ofdata storage sleds. At least one of the head nodes of the data storageunit implements a local control plane for the data storage unit,wherein, in response to a volume creation request, the local controlplane is configured to receive an assignment from a zonal control planeto service a volume requested by the volume creation request, whereinthe zonal control plane at least partially controls storage operationsrelated to the data storage unit and one or more additional data storageunits. Furthermore, the plurality of head nodes of the data storage unitare configured to service read requests and write requests directed toone or more volumes serviced by the data storage unit independent of thezonal control plane. For example, once a volume is created on aparticular data storage unit in response to a zonal control planereceiving a volume creation request, the data storage unit may serviceread and write requests directed to the volume independent of the zonalcontrol plane.

According to one embodiment, a method includes receiving, from a zonalcontrol plane by a local control plane of a data storage unit, anassignment of a volume to be serviced by the data storage unit, whereinthe data storage unit comprises a plurality of head nodes and aplurality of data storage sleds, wherein at least one of the head nodesimplements the local control plane of the data storage unit. The methodfurther includes assigning, by the local control plane, a particular oneof the head nodes of the data storage unit to function as a primary headnode for the volume; assigning, by the local control plane, anotherparticular one of the head nodes of the data storage unit to function asa secondary head node from the volume; and in response to a read orwrite request for the volume, servicing, by the primary head node of thedata storage unit, the read or write request independent of the zonalcontrol plane.

According to one embodiment a data storage system includes a pluralityof head nodes and a plurality of data storage sleds. Each of the datastorage sleds includes multiple mass storage devices and a sledcontroller for the plurality of mass storage devices mounted in the datastorage sled. Respective ones of the head nodes are configured to obtaincredentials for accessing particular portions of the mass storagedevices of respective ones of the plurality of data storage sleds. Forexample, a head node may receive a credential from a local control planeimplemented on one or more of the head nodes of the data storage unit ormay receive credentials from a zonal control plane implemented on one ormore computing devices external to the data storage unit. Each of therespective sled controllers, in response to a request from a particularhead node to write data on a particular portion of a particular massstorage device in a particular data storage sled that includes therespective sled controller, is configured to determine whether acredential included with the write request from the particular head nodeis a valid credential for accessing the particular portion of theparticular mass storage device. In response to determining thecredential is a valid credential for the particular portion of theparticular mass storage device, the respective sled controller isconfigured to cause the requested write to be performed on theparticular portion of the particular mass storage device. Also, inresponse to determining the credential is an invalid credential for theparticular portion of the particular mass storage device, the respectivesled controller is configured to decline to perform the requested writeand return a message to the particular head node indicating thecredential for accessing the particular portion of the particular massstorage device is an invalid credential. For example, if a credentialfor writing to a particular portion of a mass storage device is issuedto a head node functioning as a primary head node for a volume andanother head node of the data storage unit attempts to write to theparticular portion of the mass storage device without a credential orwith an inferior credential that is inferior to the credential held bythe primary head node, the sled controller of the data storage sled mayenforce the fencing off of the particular portion of the mass storagedevice for the head node functioning as the primary head node for thevolume by refusing to perform the write requested by the other head nodeof the data storage unit. Also, in some embodiments, a head nodefunctioning as a primary head node may determine that it has beensuperseded as primary head node by another head node of a data storageunit in response to a write request being denied by a sled controller.Such a scheme may prevent corruption of data caused by a head nodeattempting to write to a particular portion of a mass storage deviceafter another head node of a data storage unit has taken over as primaryhead node and assumed exclusive responsibility for writing new data tothe particular portion of the mass storage device.

According to one embodiment, a method includes receiving, by a head nodeof a data storage system, a write request from a client of the datastorage system; writing, by the head node, data included with the writerequest to a data storage of the head node; and requesting, by the headnode, to write the data included with the write request to a pluralityof mass storage devices in a plurality of data storage sleds of the datastorage system. Requesting to write the data to the data storage sledsincludes presenting respective credentials to respective sledcontrollers of each of the plurality of data storage sleds, wherein therespective sled controllers cause the data to be written to therespective mass storage devices of the respective data storage sleds inresponse to determining the respective credentials are valid credentialsfor accessing respective portions of the respective mass storagedevices.

According to one embodiment, a method includes determining, by a sledcontroller of a data storage system, whether a credential included witha write request from a particular head node of the data storage systemis a valid credential for accessing a particular portion of a particularmass storage device included in a sled with the sled controller. Themethod further includes in response to determining the credential is avalid credential for the particular portion of the particular massstorage device, causing the requested write to be performed on theparticular portion of the particular mass storage device. The methodalso includes determining, by a sled controller of a data storagesystem, whether another credential included with another write requestfrom another particular head node of the data storage system is a validcredential for accessing the particular portion of the particular massstorage device included in the sled with the sled controller; and inresponse to determining the other credential is an invalid credentialfor the particular portion of the particular mass storage device,declining to perform the requested write and returning a message to theother particular head node indicating the other credential for accessingthe particular portion of the particular mass storage device is aninvalid credential.

According to one embodiment, a data storage system comprises a pluralityof head nodes, for example mounted on a rack, a plurality of datastorage sleds, and at least two networking devices. The at least twonetworking devices are configured to implement at least two redundantnetworks within the data storage system, wherein to implement the atleast two redundant networks each respective head node is coupled toeach of the plurality of data storage sleds via a first one of the atleast two networking devices, each respective head node is also coupledto each of the plurality of data storage sleds via a second one of theat least two networking devices, and each respective head node isassigned at least two unique network addresses for communicating withthe plurality of data storage sleds. For example, a particular head nodeof a data storage unit may be configured to communicate with externaldevices via a first path through the first networking device and using afirst address, such as a first IP address and also communicate with theexternal device via a redundant network path through the secondnetworking device and using a second address, such as a second IPaddress. Also, a head node may be configured to communicate with massstorage devices in separate ones of the data storage sleds mounted inthe rack via a first path through the first networking device andthrough a second path through the second networking device. In someembodiments, a data storage unit may be configured such that only asingle network hop is required for a head node to retrieve data storedin data storage sleds of the data storage unit.

According to one embodiment, a data storage system includes a head nodecomprising at least three network interfaces, wherein at least two ofthe network interfaces are configured to implement at least tworedundant networks within the data storage system and at least onenetwork interface of the at least three network interfaces is configuredto enable communications between the data storage system and externalclients. To implement the at least two redundant networks the at leasttwo network interfaces of the head node are configured to couple to eachof a plurality of data storage sleds via a first networking device andcouple to each of the plurality of data storage sleds via a secondnetworking device. Also, the head node is assigned at least two uniquenetwork addresses for communicating with the plurality of data storagesleds.

According to one embodiment, a data storage system includes a datastorage sled comprising at least two network interfaces configured toimplement at least two redundant networks within the data storagesystem, wherein to implement the at least two redundant networks, the atleast two network interfaces of the data storage sled are configured tocouple to a particular head node via a first a networking device andcouple to the same particular head node via a second networking device.Also, the data storage sled is assigned at least two unique networkaddresses for communicating with the particular head node or one or moreadditional head nodes of the data storage system.

Some data storage systems, such as storage area networks (SAN) may allowa server or a pair of servers to access a shared set of storageresources. However, such systems may be susceptible to significantlosses in performance due to a server failure. Also, in such systems,data may be durably stored in storage devices of the SAN network, butnot durably stored in the servers accessing the SAN network.

In order to provide high durability data storage and low latencies foraccessing data, a data storage unit may store data in local storages ofhead nodes that function as servers for the data storage system,replicate the data to another head node of the data storage unit, andalso store the data across multiple mass storage devices in multipledata storage sleds of the data storage unit. Thus, a data storage systemthat includes a data storage unit may provide low latency input/outputoperations for data stored in a storage of a head node, while stillproviding data durability due to the data being replicated to anotherhead node. Furthermore, the data storage system may provide even higherdurability for the data once the data is stored in multiple mass storagedevices in different data storage sleds of the data storage unit. Thus,a data storage system may provide varying levels of data durability andinput/output operation latency depending on whether the data is storedin a storage of a head node and replicated to another head node orwhether the data is stored in multiple mass storage devices in differentdata storage sleds of the data storage system.

In some embodiments, data may be initially stored in a storage of a headnode and replicated to a storage of another head node, and may beasynchronously moved to multiple mass storage devices in different datastorage sleds that form a RAID array (random array of independent disks)to store the data. In some embodiments, recently stored data orfrequently accessed data may remain in a head node storage to allow forlow latency access to the data. The data may then be moved to massstorage devices in data storage sleds of a data storage unit of the datastorage system after a certain amount of time has elapsed since the datawas last accessed or stored. Moving the data to the mass storage devicesmay increase the durability of the data as compared to being stored in astorage of a primary head node and being replicated to a storage of asecondary head node. Thus a data storage system may provide differentlevels of durability and latency based on a staleness or a frequency ofaccess to data stored in the data storage system. In some embodiments,other criteria may be used to determine when data stored in a storage ofa head node is to be moved to mass storage devices of data storage sledsof a data storage unit. For example, data may be collected in a log of ahead node and upon an amount of data being stored in the log exceeding athreshold amount, the data may be moved to mass storage devices of datastorage sleds of a data storage unit of the data storage system.

In some embodiments, a data storage unit of a data storage system maymultiple head nodes, multiple data storage sleds, and at least twonetworking devices. The data storage unit may further include connectorsfor coupling the data storage unit with at least two separate powersources. The data storage unit may also include at least two powerdistribution systems within the data storage unit to provide redundantpower to the head nodes, the data storage sleds, and the networkingdevices of the data storage unit. Furthermore, the at least twonetworking devices of the data storage unit may implement at least tworedundant networks within the data storage unit that enablecommunications between the head nodes of the data storage unit and thedata storage sleds of the data storage unit. Furthermore, the at leasttwo networking devices of the data storage unit may implement at leasttwo redundant networks within the data storage unit that enablecommunications between the head nodes of the data storage unit andexternal clients of the data storage unit. In some embodiments, a datastorage unit that include redundant networks and redundant power mayprovide high reliability and data durability for data storage and accesswhile storing data locally within devices mounted within a single rack.

In some embodiments, a data storage unit of a data storage system mayinclude multiple head nodes that are assigned network addresses that areroutable from devices external to the data storage unit. Thus, externalclients may communicate directly with head nodes of a data storage unitwithout the communications being routed through a control plane of thedata storage system that is external to the data storage unit, such as azonal control plane. Also, a data storage system that includes multipledata storage units may implement a zonal control plane that assignsvolumes or volume partitions to particular ones of the data storageunits of the data storage system. Also, a zonal control plane maycoordinate operations between data storage units, such as rebalancingloads by moving volumes between data storage units. However, a datastorage unit may also implement a local control plane configured toperform fail over operations for head nodes and mass storage devices ofdata storage sleds of the data storage unit. Because head nodes of adata storage unit may communicate directly with client devices andbecause a local control plane may manage fail over operations within adata storage unit, the data storage unit may operate autonomouslywithout relying on a zonal control plane once a volume has been createdon the data storage unit.

In some embodiments, in order to prevent corruption of data stored inmass storage devices of a data storage system, a data control plane maybe at least partially implemented on a sled controller of a data storagesled of the data storage system. The data storage sled may includemultiple mass storage devices serviced by the sled controller. Also,portions of respective mass storage devices of a particular data storagesled may be reserved for a particular volume serviced by a particularhead node functioning as a primary head node for the particular volume.In order to reserve the portions for the particular volume or a volumepartition of the particular volume, a sled controller of a data storagesled may provide a token to a head node requesting to reserve theportions. Once the portions are reserved for the particular volume bythe head node acting as the primary head node, the head node whileacting as a primary head node for the particular volume, may provide thetoken to the sled controller along with a write request when writing newdata to the portions. The sled controller may verify the token anddetermine the head node is authorized to write to the portions. Also,the sled controller may be configured to prevent writes from head nodesthat are not authorized to write to the particular portions of the massstorage devices of the data storage sled that includes the sledcontroller. The sled controller may refuse to perform a write requestbased on being presented an invalid token or based on a token not beingincluded with a write request.

In some embodiments, a control plane such as a local control plane or azonal control plane of a data storage system may issue unique sequencenumbers to head nodes of the data storage system to indicate which headnode is a primary head node for a particular volume or volume partition.A primary head node may present a sequence number issued from a controlplane to respective ones of the sled controllers of respective ones ofthe data storage sleds to reserve, for a particular volume or volumepartition, respective portions of mass storage devices serviced by therespective ones of the respective sled controllers. In response, thesled controllers may issue a token to the primary head node to beincluded with future write requests directed to the respective portions.

In order to facilitate a failover operation between a primary head nodeand a secondary head node, a control plane may issue new credentials,e.g. a new sequence number, to a head node assuming a role of primaryhead node for a volume or volume partition. The newly assigned primaryhead node may present the credentials, e.g. new sequence number, torespective sled controllers to receive respective tokens that supersedetokens previously used to a previous head node acting as a primary headnode for a particular volume or volume partition that had data stored inportions of mass storage devices service by the sled controller. Thus,during a fail over event, a previous primary head node may be fenced offfrom portions of mass storage devices to prevent corruption of datastored on the mass storage devices during the failover event.

FIG. 1 illustrates a data storage unit comprising head nodes and datastorage sleds, according to some embodiments. Data storage unit 100,which may be included in a data storage system, includes networkswitches 102 and 104, head nodes 106 and data storage sleds 134-144 onshelves 118. Each data storage sled 134-144 includes a sled controller112 and mass storage devices 110. The head nodes 106, data storage sleds134-144, and network switches 102 and 104 are mounted in rack 130. Insome embodiments, networking devices, such as network switches 102 and104, may be mounted in a position adjacent to and external from a rackof a data storage unit, such as rack 130 of data storage unit 100. Adata storage unit may have redundant network connections to a networkexternal to the data storage unit, such as network 128 that is connectedto both network switch 102 and network switch 104. In some embodiments,components of a data storage unit, such as network switches 102 and 104,head nodes 106, and data storage sleds 134-144 may be connected toredundant power sources. For example, power connections 108 indicatepower connections for network switches 102 and 104, head nodes 106, anddata storage sleds 134-144. Note that power connections 108 areillustrated as a power symbol for simplicity of illustration, but mayinclude various types of power connectors and power distributionsystems. For example, power connectors of data storage unit components,such as head nodes and data storage sleds, may couple to dual powerdistribution systems within a data storage unit that receive power fromdual power sources. In some embodiments, a data storage unit may includemore than two redundant power distribution systems from more than tworedundant power sources.

Each head node of a data storage unit, such as each of head nodes 106,may include a local data storage and multiple network interface cards.For example, a head node may include four network ports, wherein twonetwork ports are used for internal communications with data storagesleds of a data storage unit, such as data storage sleds 134-144, andtwo of the network ports are used for external communications, forexample via network 128. In some embodiments, each head node may beassigned two publicly routable network addresses that are routable fromclient devices in network 128 and may also be assigned two local networkaddresses that are local to a data storage unit and are routable forcommunications between the head node and data storage sleds of the datastorage unit. Thus, a data storage unit, such as data storage unit 100,may include multiple redundant networks for communications within thedata storage unit. In some embodiments, publicly routable networkaddresses may be used for internal communications between head nodes anddata storage sleds and a head node may be assigned four publiclyroutable network addresses that are routable from client devices innetwork 128. The data storage unit may also include redundant powerdistribution throughout the data storage unit. These redundancies mayreduce risks of data loss or downtime due to power or network failures.Because power and network failure risks are reduced via redundant powerand network systems, volumes may be placed totally or at least partiallywithin a single data storage unit while still meeting customerrequirements for reliability and data durability.

Also, one or more head nodes of a data storage unit, such as one or moreof head nodes 106, may function as a head node and additionallyimplement a local control plane for a data storage unit. In someembodiments, a local control plane may be implemented in a logicalcontainer separate from other control and storage elements of a headnode. A local control plane of a data storage unit may select amongstany of the head nodes, such as any of head nodes 106, of the datastorage unit when selecting a head node to designate as a primary headnode for a volume or volume partition and may select amongst any of theremaining head nodes of the data storage unit when selecting a head nodeto designate as a secondary head node for the volume or volumepartition. For example a first one of head nodes 106 may be designatedas a primary head node for a volume or volume partition and any of theremaining head nodes 106 may be selected as a secondary head node forthe volume or volume partition. In some embodiments, a given one of thehead nodes 106 may be designated as a primary head node for a givenvolume or volume partition and may also be designated as a secondaryhead node for another volume or volume partition.

Additionally, any head node may be assigned or select columns of spaceon mass storage devices in any of the data storage sleds of a datastorage unit for storing data for a particular volume or volumepartition. For example, any of head nodes 106 may reserve columns ofspace in mass storage devices 110 in any of data storage sleds 134-144.However, any particular column of space of a mass storage device mayonly be assigned to a single volume or volume partition at a time.

Because multiple head nodes and multiple data storage sleds areavailable for selection, a failure of a particular head node or afailure of a mass storage device in a particular data storage sled maynot significantly reduce durability of data stored in the data storageunit. This is because, upon failure of a head node, a local controlplane may designate another head node of the data storage unit tofunction as secondary head node for a volume or volume partition. Thus,the volume is only without a secondary head node for a short period oftime during which a new secondary head node is being designated andindex data is being replicated from the primary head node to thesecondary head node. Furthermore, when a head node of a data storageunit fails, other head nodes of the data storage unit may still be ableto access data in all of the storage sleds of the data storage unit.This is because no single data storage sled is exclusively assigned toany particular head node, but instead columns of space on individualmass storage devices of the data storage sleds are assigned toparticular head nodes for particular volumes or volume partitions. Thisarrangement greatly reduces the blast radius of a head node failure or adisk failure as compared to other storage systems in which each serverhas a dedicated set of storage devices.

As discussed in more detail below, in some embodiments, a head node orlocal control plane of a data storage unit may be configured toreplicate data stored on mass storage devices that are located in a datastorage sled to other mass storage devices in other data storage sleds.Thus, for example, when a data storage sled with a failed mass storagedevice is removed from a data storage unit for replacement or repair,data from one or more non-failed mass storage devices in a data storagesled may still be available because the data has been replicated toother data storage sleds of the data storage unit. For example, if asingle mass storage device 110 in data storage sled 134 failed, datastored in the remaining mass storage devices 110 of data storage sled134 may be replicated to mass storage devices 110 in any of data storagesleds 136-144. Thus while data storage sled 134 is removed from datastorage unit 100 for repair or replacement of the failed mass storagedevice 110, data previously stored on the non-failed mass storagedevices 110 of data storage sled 134 may still be available to headnodes 106.

Also, a data storage unit, such as data storage unit 100, may performread and write operations independent of a zonal control plane. Forexample, each of head nodes 106 may be assigned one or more networkaddresses, such as IP addresses, that are advertised outside of datastorage unit 100. Read and write requests may be routed to individualhead nodes at the assigned network addresses of the individual headnodes via networking devices of the data storage unit, such as networkswitches 102 and 104, without the read and write requests being routedthrough a control plane external to the data storage unit, such as acontrol plane external to data storage unit 100.

In some embodiments, a data storage sled, such as one of data storagesleds 134-144, may include a sled controller, such as one of sledcontrollers 112. A sled controller may present the mass storage devicesof the data storage sled to the head nodes as storage destinationtargets. For example head nodes and data storage sleds may be connectedover an Ethernet network. In some embodiments, head nodes, such as headnodes 106 may communicate with mass storage devices 110 and vice versavia sled controllers 112 using a Non-volatile Memory Express (NVMe)protocol, or other suitable protocols. In some embodiments, each headnode may be assigned multiple private network addresses forcommunication with data storage sleds over redundant internal Ethernetnetworks internal to a data storage unit. In some embodiments, a headnode at an I/O processing software layer may perform a local diskoperation to write or read from a mass storage device of a data storagesled and another software layer of the head node may encapsulate orconvert the I/O operation into an Ethernet communication that goesthrough a networking device of the data storage unit to a sledcontroller in one of the data storage sleds of the data storage unit. Anetwork interface of a head node may be connected to a slot on amotherboard of the head node, such as a PCIe slot, so that the massstorage devices of the data storage sleds appears to the operatingsystem of the head node as a local drive, such as an NVMe drive. In someembodiments, a head node may run a Linux operating system or other typeof operating system. The operating system may load standard drivers,such as NVMe drivers, without having to change the drivers tocommunicate with the mass storage devices mounted in the data storagesleds.

In some embodiments, a local control plane may be configured todesignate more than one head node as a secondary/back-up head node for avolume or a volume partition and also adjust a number of mass storagedevices that make up a RAID array for longer term storage of data forthe data volume or volume partition. Thus if increased durability isdesired for a particular volume or volume partition, the volume data maybe replicated on “N” head nodes and subsequently stored across “M” massstorage devices in data storage sleds of the data storage unit, whereinthe number “N” and the number “M” may be adjusted to achieve aparticular level of durability. In some embodiments, such an arrangementmay allow high levels of durability to be realized without having tostore data for a data volume outside of a single data storage unit.Also, in such an arrangement, input/output operations may be performedmore quickly because data for a particular volume is stored within asingle data storage unit.

Also, a given head node may be designated as a primary head node or asecondary head node for multiple volumes. Furthermore, a zonal controlplane of a data storage system or a local control plane of a datastorage unit may balance volume placement across head nodes of a datastorage unit. Because volumes are distributed amongst the head nodes,variations in peak IOPS to average IOPS may be reduced because while onevolume may experience peak load other volumes serviced by a particularhead node may experience less than peak IOPS load. In some embodiments,a zonal or local control plane may adjust head node designations orvolume assignments to balance loads if volumes on a particular head nodeexperience significantly more IOPS than volumes serviced by other headnodes.

While, FIG. 1 illustrates mass storage devices 110 as solid statedrives, any suitable storage device may be used. For example, in someembodiments, storage devices 110 may include hard disk drives. Also,FIG. 1 illustrates networking devices 102 and 104 to be networkingswitches. However, in some embodiments, other suitable networkingdevices may be used such as routers, etc.

In some embodiments, a data storage unit, such as data storage unit 100,may be part of a larger provider network system. Also, in someembodiments more than one data storage unit may be included in a blockstorage service of a provider network. For example, FIG. 2 illustratessuch an example provider network, according to some embodiments.

FIG. 2 is a block diagram illustrating a provider network that includesmultiple network-based services such as a block-based storage servicethat implements dynamic resource creation to connect with clientresources, according to some embodiments. Provider network 200 may beset up by an entity such as a company or a public sector organization toprovide one or more services (such as various types of cloud-basedcomputing or storage) accessible via the Internet and/or other networksto clients 210. Provider network 200 may include numerous data centershosting various resource pools, such as collections of physical and/orvirtualized computer servers, storage devices, networking equipment andthe like (e.g., computing device 2100 described below with regard toFIG. 21), needed to implement and distribute the infrastructure andservices offered by the provider network 200. In some embodiments,provider network 200 may provide computing resources, such as virtualcompute service 240, storage services, such as block-based storageservice 220, and/or any other type of network-based services 260.Clients 210 may access these various services offered by providernetwork 200 via network 270. Likewise network-based services maythemselves communicate and/or make use of one another to providedifferent services. For example, computing resources offered to clients210 in units called “instances,” such as virtual or physical computeinstances, may make use of particular data volumes 226, providingvirtual block-based storage for the compute instances. Also, note thatany of the data storage units 224 a, 224 b, 224 n may be data storageunits such as data storage unit 100 illustrated in FIG. 1.

As noted above, virtual compute service 240 may offer various computeinstances, such as compute instances 254 a and 254 b to clients 210. Avirtual compute instance may, for example, comprise one or more serverswith a specified computational capacity (which may be specified byindicating the type and number of CPUs, the main memory size, and so on)and a specified software stack (e.g., a particular version of anoperating system, which may in turn run on top of a hypervisor). Anumber of different types of computing devices may be used singly or incombination to implement the compute instances of virtual computeservice 240 in different embodiments, including special purpose computerservers, storage devices, network devices and the like. In someembodiments instance clients 210 or any other user may be configured(and/or authorized) to direct network traffic to a compute instance. Invarious embodiments, compute instances may mount, connect, attach or mapto one or more data volumes 226 provided by block-based storage service220 in order to obtain persistent block-based storage for performingvarious operations.

Compute instances may operate or implement a variety of differentplatforms, such as application server instances, Java™ virtual machines(JVMs), special-purpose operating systems, platforms that supportvarious interpreted or compiled programming languages such as Ruby,Perl, Python, C, C++ and the like, or high-performance computingplatforms) suitable for performing client applications, without forexample requiring the client 210 to access an instance.

Compute instance configurations may also include compute instances witha general or specific purpose, such as computational workloads forcompute intensive applications (e.g., high-traffic web applications, adserving, batch processing, video encoding, distributed analytics,high-energy physics, genome analysis, and computational fluid dynamics),graphics intensive workloads (e.g., game streaming, 3D applicationstreaming, server-side graphics workloads, rendering, financialmodeling, and engineering design), memory intensive workloads (e.g.,high performance databases, distributed memory caches, in-memoryanalytics, genome assembly and analysis), and storage optimizedworkloads (e.g., data warehousing and cluster file systems). Size ofcompute instances, such as a particular number of virtual CPU cores,memory, cache, storage, as well as any other performance characteristic.Configurations of compute instances may also include their location, ina particular data center, availability zone, geographic, location, etc.,and (in the case of reserved compute instances) reservation term length.

As illustrated in FIG. 2, a virtualization host, such as virtualizationhosts 242 a and 242 n, may implement and/or manage multiple computeinstances 252 a, 252 b, 254 a, and 254 b respectively, in someembodiments, and may be one or more computing devices, such as computingdevice 2100 described below with regard to FIG. 21. Virtualization hosts242 may also provide multi-tenant hosting of compute instances. Forexample, in some embodiments, one virtualization host may host a computeinstance for one entity (e.g., a particular client or account of virtualcomputing service 210), while another compute instance hosted at thesame virtualization host may be hosted for another entity (e.g., adifferent account). A virtualization host may include a virtualizationmanagement module, such as virtualization management modules 244 a and244 b capable of instantiating and managing a number of differentclient-accessible virtual machines or compute instances. Thevirtualization management module may include, for example, a hypervisorand an administrative instance of an operating system, which may betermed a “domain-zero” or “dom0” operating system in someimplementations. The dom0 operating system may not be accessible byclients on whose behalf the compute instances run, but may instead beresponsible for various administrative or control-plane operations ofthe network provider, including handling the network traffic directed toor from the compute instances.

Virtual computing service 240 may implement control plane 250 to performvarious management operations. For instance, control plane 250 mayimplement resource management to place compute instances, and manage theaccess to, capacity of, mappings to, and other control or direction ofcompute instances offered by provider network. Control plane 250 mayalso offer and/or implement a flexible set of resource reservation,control and access interfaces for clients 210 via an interface (e.g.,API). For example, control plane 250 may provide credentials orpermissions to clients 210 such that compute instance controloperations/interactions between clients and in-use computing resourcesmay be performed.

In various embodiments, control plane 250 may track the consumption ofvarious computing instances consumed for different virtual computerresources, clients, user accounts, and/or specific instances. In atleast some embodiments, control plane 250 may implement variousadministrative actions to stop, heal, manage, or otherwise respond tovarious different scenarios in the fleet of virtualization hosts 242 andinstances 252, 254. Control plane 250 may also provide access to variousmetric data for client(s) 210 as well as manage client configuredalarms.

In various embodiments, provider network 200 may also implementblock-based storage service 220 for performing storage operations.Block-based storage service 220 is a storage system, composed of one ormore computing devices implementing a zonal control plane 230 and a poolof multiple data storage units 224 a, 224 b through 224 n (e.g., datastorage units such as data storage unit 100 illustrated in FIG. 1),which provide block level storage for storing one or more sets of datavolume(s) 226 a, 226 b through 226 n. Data volumes 226 may be attached,mounted, mapped, or otherwise connected to particular clients (e.g., avirtual compute instance of virtual compute service 240), providingvirtual block-based storage (e.g., hard disk storage or other persistentstorage) as a contiguous set of logical blocks. In some embodiments, adata volume 226 may be divided up into multiple data chunks orpartitions (including one or more data blocks) for performing otherblock storage operations, such as snapshot operations or replicationoperations. A volume snapshot of a data volume 226 may be a fixedpoint-in-time representation of the state of the data volume 226. Insome embodiments, volume snapshots may be stored remotely from a datastorage unit 224 maintaining a data volume, such as in another storageservice 260. Snapshot operations may be performed to send, copy, and/orotherwise preserve the snapshot of a given data volume in anotherstorage location, such as a remote snapshot data store in other storageservice 260. In some embodiments, a block-based storage service, such asblock-based storage service 220, may store snapshots of data volumesstored in the block-based storage service.

Block-based storage service 220 may implement zonal control plane 230 toassist in the operation of block-based storage service 220. In variousembodiments, zonal control plane 230 assists in creating volumes on datastorage units 224 a, 224 b, through 224 n and moving volumes betweendata storage units 224 a, 224 b, through 224 n. In some embodiments,access to data volumes 226 may be provided over an internal networkwithin provider network 200 or externally via network 270, in responseto block data transaction instructions.

Zonal control plane 230 may provide a variety of services related toproviding block level storage functionality, including the management ofuser accounts (e.g., creation, deletion, billing, collection of payment,etc.). Zonal control plane 230 may implement capacity management, whichmay generate and manage a capacity model for storage service 220, andmay direct the creation of new volumes on particular data storage unitsbased on the capacity of storage service 220. Zonal control plane 230may further provide services related to the creation and deletion ofdata volumes 226 in response to configuration requests.

Clients 210 may encompass any type of client configured to submitrequests to network provider 200. For example, a given client 210 mayinclude a suitable version of a web browser, or may include a plug-inmodule or other type of code module configured to execute as anextension to or within an execution environment provided by a webbrowser. Alternatively, a client 210 may encompass an application suchas a database application (or user interface thereof), a mediaapplication, an office application or any other application that maymake use of compute instances, a data volume 226, or other network-basedservice in provider network 200 to perform various operations. In someembodiments, such an application may include sufficient protocol support(e.g., for a suitable version of Hypertext Transfer Protocol (HTTP)) forgenerating and processing network-based services requests withoutnecessarily implementing full browser support for all types ofnetwork-based data. In some embodiments, clients 210 may be configuredto generate network-based services requests according to aRepresentational State Transfer (REST)-style network-based servicesarchitecture, a document- or message-based network-based servicesarchitecture, or another suitable network-based services architecture.In some embodiments, a client 210 (e.g., a computational client) may beconfigured to provide access to a compute instance or data volume 226 ina manner that is transparent to applications implemented on the client210 utilizing computational resources provided by the compute instanceor block storage provided by the data volume 226.

Clients 210 may convey network-based services requests to providernetwork 200 via external network 270. In various embodiments, externalnetwork 270 may encompass any suitable combination of networkinghardware and protocols necessary to establish network-basedcommunications between clients 210 and provider network 200. Forexample, a network 270 may generally encompass the varioustelecommunications networks and service providers that collectivelyimplement the Internet. A network 270 may also include private networkssuch as local area networks (LANs) or wide area networks (WANs) as wellas public or private wireless networks. For example, both a given client210 and provider network 200 may be respectively provisioned withinenterprises having their own internal networks. In such an embodiment, anetwork 270 may include the hardware (e.g., modems, routers, switches,load balancers, proxy servers, etc.) and software (e.g., protocolstacks, accounting software, firewall/security software, etc.) necessaryto establish a networking link between given client 210 and the Internetas well as between the Internet and provider network 200. It is notedthat in some embodiments, clients 210 may communicate with providernetwork 200 using a private network rather than the public Internet.

Data Replication

FIG. 3 is a block diagram illustrating head nodes and data storage sledsof a data storage unit storing block storage data in response to a writerequest, according to some embodiments. Head nodes 306 illustrated inFIG. 3 may be the same as head nodes 106 illustrated in FIG. 1. Also,data storage sleds 326 may be the same as data storage sleds 134-144illustrated in FIG. 1.

As discussed above, a data storage system that includes a data storageunit, may store volume data in a data storage of a first head nodedesignated as a primary head node for a volume or volume partition andmay also replicate the volume data to a second head node designated as asecondary head node for the volume or volume partition. For example, attime 1, a write request 302 is routed to head node 306 that isdesignated as a primary head node for a volume or volume partition. Attime 2 subsequent to the write request being received at head node 306,data included with the write request is stored in storage 314 of primaryhead node 306 and primary head node 306 causes the data included withthe write request to be replicated to storage 316 of secondary head node308. Replication of the data to secondary head node 306 is performedconcurrently or nearly concurrently with storing the data in storage 314of primary head node 306. Also, as shown in FIG. 3 at time 2,replication of the data to the secondary head node may include thesecondary head node sending an acknowledgment back to the primary headnode indicating that the data has been replicated to the secondary headnode. Subsequently at time 3, which is also nearly concurrent with thedata being stored in the storage of the primary head node and the databeing replicated to the secondary head node, the primary head node, headnode 306, may issue an acknowledgement 320 to the client device thatrequested write 302 has been committed in data storage system 300.

In some embodiments, a write request, such as write request 302, may beconcurrently received at a primary head node and a secondary head node.In such embodiments, the primary head node may verify that the secondaryhead node has committed the write before acknowledging at time 3 thatthe write has been committed in the data storage system.

At a later point in time 4, e.g. asynchronous to times 1-3, the primaryhead node, e.g. head node 306, may cause data stored in storage 314,that includes the data included with the write request and that mayinclude additional data stored before or after the write request, to beflushed to mass storage devices 322 of the data storage sleds 326 of thedata storage unit. For example, at time 4 data is flushed to massstorage devices 322 of data storage sleds 326. In some embodiments, datais divided into portions and stored across multiple mass storagedevices, each in a different sled and/or on a different shelf of a datastorage unit. In some embodiments, data is also erasure encoded whenstored in mass storage devices of data storage sleds. For example, dataflushed from storage 314 of head node 306 may be divided into sixportions where each portion is stored in a different mass storage deviceof a different data storage sled on a different shelf of a data storageunit 350 of data storage system 300 and is also erasure encoded acrossthe different mass storage devices. For example data portions are storedin sled A of shelf 1, sled B of shelf 2, sled A of shelf 3, sled C ofshelf 4, sled B of shelf 5, and sled C of shelf 6.

Also, as can be seen in FIG. 3, a data storage unit, such as datastorage unit 350, may include “M” number of shelves and “N” number ofhead nodes. The portions of data may be stored on portions of massstorage devices 322 in the respective data storage sleds 326. In orderto distinguish between a portion of data and a portion of space on amass storage device, a portion of space on a mass storage device may bereferred to herein as a “column” of a mass storage device. Furthermore,a set of columns of mass storage devices that store different portionsof data of a volume such as the columns shown in sled A of shelf 1, sledB of shelf 2, sled A of shelf 3, sled C of shelf 4, sled B of shelf 5,and sled C of shelf 6 may collectively make up what is referred toherein as an “extent.” For example, in an erasure encoded RAID sixarray, an extent may include six columns that collectively make up theRAID array. Four of the columns may store striped data and two of thecolumns may store parity data. In some embodiments, other replicationalgorithms other than erasure encoding may be used such as quorumalgorithms, etc.

In some embodiments, each column of an extent may be in a differentfault domain of a data storage unit. For example, for the extent beingstored in FIG. 3 each column is located in a different data storage sledthat is mounted on a different shelf of the data storage unit 350. Thusa failure of a sled controller, such as one of sled controllers 324, mayonly affect a single column. Also if a power supply of a data storagesled fails it may only affect a single data storage sled or if a part ofa power distribution system fails it may affect a single shelf. However,because each column of an extent may be located in a different shelf, ashelf level power event may only affect a single column of the extent.

In some embodiments, a head node of a data storage unit, such as one ofhead nodes 304, 306, 308, or 310, may implement a local control plane.The local control plane may further implement an extent allocationservice that allocates extents to head nodes designated as a primaryhead node for a volume or volume partition. In some embodiments, anextent allocation service may allocate a set of extents to a particularvolume referred to herein as a “sandbox.” The primary head node for theparticular volume may then select extents to store data on during a dataflush from the primary head node to data storage sleds of the datastorage unit by selecting an extent from the sandbox allocated for theparticular volume.

In some embodiments, if insufficient space is available in theparticular volume's sandbox or if a particular placement would cause adata durability of data to be saved to fall below a minimum requireddurability for the particular volume, a primary head node for theparticular volume may select columns outside of the particular volume'ssandbox to write data for the particular volume. For example, a sandboxmay include multiple columns that make up multiple extents in differentones of the data storage sleds 326 on different ones of the shelves of adata storage unit 350. A primary head node may be able to flush data tocolumns within a particular volume's sandbox without having to requestextent allocation from a local control plane that implements an extentallocation service. This may further add durability and reliability to adata storage unit because a primary head node for the particular volumemay continue to flush data even if communication is lost with a localcontrol plane within the data storage unit. However, if space is notavailable or a placement would cause durability for a particular volumeor volume partition to fall below a minimum threshold, a primary headnode may flush data to columns outside of the particular volume'ssandbox. In some embodiments, a primary head for a particular volume mayflush data to columns outside the primary head node's sandbox withoutrequesting an allocation from a local control plane that implements anextent allocation service. For example, a primary head node may storeaddresses for each sled controller in a data storage unit and may flushdata to any sled controller in the data storage unit that is associatedwith mass storage devices with available columns.

As will be discussed in more detail in regard to FIG. 15, a sledcontroller of a data storage sled, such as sled controller 324, mayimplement a fencing protocol that prevents a primary head node fromwriting to columns for which another primary head node has assumedcontrol after the primary head node has been superseded by another headnode assuming the role of primary head node for a particular volume orvolume partition. It should be pointed out that a secondary head node orother back-up head nodes may not flush data to data storage sleds andflushing may be limited to only being performed by a primary head node.

Because for a particular volume, the volume's data may be stored in astorage of a primary head node and replicated to a secondary head nodeand may later be moved to being stored across an extent of mass storagedevices in different data storage sleds of a data storage unit, an indexwith pointers to where the data is stored may be used for subsequentread requests and write requests to locate the data. Also in someembodiments, storages of a head node may be log-structured such thatincoming write request are written to the head of the log of the headnode's log-structured storage. An index entry may be added indicatingwhere the written data is stored in the head node's log and subsequentlythe index may be updated when the written data is flushed from the logof the primary head node to an extent comprising columns of mass storagedevices of the data storage system.

FIGS. 4A-4B are block diagrams illustrating a log-structured storage andan index of a head node storage, according to some embodiments. Headnode 402 includes storage 404 that includes log 408 and index 406.Volume data may be stored in log 408 prior to being flushed to massstorage devices of a data storage unit. Index information 410 mayinclude an entry for the volume data and a corresponding pointer towhere the volume data is stored. For example, index information 410indicates that data for volume 1, offset A, length B is stored in logstorage 408 at log segment C and offset D. In some embodiments, a log ofa head node such as log 408 of storage 404 of head node 402 may storedata for more than one volume. For example, index information 410 alsoincludes an entry for volume 2 offset E, length F and a correspondingpointer indicating the data for this volume entry is stored in log 408at log segment G, offset H.

While FIGS. 4A-B illustrate log storage 408 and index 406 as separatefrom each other, in some embodiments, an index, such as index 406, maylay on top of a log or side-by-side with a log, such as log storage 408.

When data for a volume is moved from a storage of a head node to beingstored in an extent across multiple mass storage devices of a datastorage unit, the data for the volume may be removed from a log of ahead node storage and an index of the head node storage may be updatedto indicate the new location at which the data for the volume is stored.For example, in FIG. 4B, index information 412 indicates that data forvolume 1, offset A, length B is now stored at extent A and data forvolume 2, offset E, length F is now stored at extent B. Note that thelabels “extent A” and “extent B” are used for ease of illustration. Insome embodiments, an index may include addresses of data storage sledswhere the data for the volume is located, such as local IP addresses ofthe data storage sleds, and addresses of the columns of the mass storagedevices within the data storage sleds. In some embodiments, an index mayinclude another label such as “extent A” where each head node storesinformation for locating “extent A” or may consult an extent allocationservice for locating “extent A.” In some embodiments, an index mayinclude addresses of data storage sleds where the data for the volume islocated and sled controllers of the data storage sleds may be able todetermine the appropriate columns based on volume IDs stored inrespective columns allocated to the volume.

When a read request is received by a head node designated as a primaryhead node for a volume, the head node may consult an index of a storageof the head node, such as index 406 of storage 404, to determine what isthe latest version of the volume's data and where the latest version ofthe volume's data is stored. For example a primary head node, such ashead node 402, may consult the primary head node's index, such as index406, to determine if the latest version of the volume's data is storedin the head node's log, such as log 408, or is stored in an extentcomprising mass storage devices of the data storage unit.

FIG. 5 illustrates a partial view of a data storage unit that storesportions of a volume partition in multiple mass storage devices inmultiple data storage sleds on multiple shelves of the data storageunit, according to some embodiments. FIG. 5 illustrates an examplestorage pattern for extent A from index 406 in FIG. 4B. Extent A fromindex 406 illustrated in FIG. 4B is shown as extent A 502 in FIG. 5Also, an example storage pattern for extent B from index 406 illustratedin FIG. 4B is shown in FIG. 5 as extent B 504. Note that a data storagesled may include multiple columns of multiple extents. Also, in someembodiments a single mass storage device may include multiple columns ofmultiple extents.

FIGS. 6A-B illustrate columns of mass storage devices storing differentportions of a volume partition, according to some embodiments. FIG. 6Aillustrates an embodiment in which data flushed to extent A, which maybe the same extent A as described in FIGS. 4 and 5, is erasure encodedacross 4+2 columns. The striped data 602 may include the original dataflushed from log 408 divided into multiple portions and the parity data604 may include encoded data that allows the flushed data to berecreated in case of failure of one or more of the mass storage devicesor sleds that include one of the columns. FIG. 6B illustrates a similarembodiment where extent B is erasure encoded across four striped datacolumns 606 and two parity columns 608. Note that in FIG. 6B the data isstored in a different location in the column than is shown in FIG. 6A.This is intended to illustrate that the columns shown in FIG. 6B mayalready store data previously written to the columns of extent B,whereas the data being written to extent A may be the first set of datawritten to extent A. Also, it is worth noting that for a particularvolume, multiple extents may be assigned to store data of the volume. Insome embodiments, an extent may represent a fixed amount of storagespace across a set number of columns of mass storage devices. When anextent is filled for a particular volume, another extent may beallocated to the volume by a head node or an extent allocation service.FIGS. 6A and 6B illustrate an example RAID level and erasure encodingtechnique. However, in some embodiments various other RAID levels may beused and various data coding techniques may be used to increasedurability of stored data. It also worth noting that erasure encodingdata may reduce a number of columns needed to achieve a particular levelof durability. For example, data stored that is not erasure encoded mayrequire the data to be stored redundantly across 8 columns to achieve agiven level of durability, whereas a similar level of durability may beachieved by erasure encoding the data across fewer columns. Thus erasureencoding data may significantly reduce an amount of storage resourcesthat are needed to store data to a particular level of durability. Forexample, data erasure encoded according to a 4+2 erasure coding schememay be recreated from any four of the six columns, wherein the sixcolumns include four columns of striped data segments and two columns ofparity data segments.

In some embodiments, a data storage system may implement one or morecommunication protocols between head nodes and data storage sleds of thedata storage system that allow for rapid communications between the headnodes and the data storage sleds. Thus, high levels of performance maybe provided to clients of a data storage system despite volume databeing erasure encoded across multiple columns of mass storage devices indifferent data storage sleds. For example, a data storage system mayimplement a protocol for reliable out-of-order transmission of packetsas described in U.S. patent application Ser. No. 14/983,436 filed onDec. 29, 2015, which is herein incorporated by reference. Also, forexample, a data storage system may implement a protocol for establishingcommunication between a user application and a target applicationwherein the network does not require an explicit connection between theuser application and the target application as described in U.S. patentapplication Ser. No. 14/983,431 filed on Dec. 29, 2015, which is hereinincorporated by reference. In some embodiments, implementation of suchprotocols may permit data erasure encoded across multiple mass storagedevices in multiple different data storage sleds to be read by a headnode in a timely manner such that, from a perspective of a client deviceof the data storage system, performance is comparable to a system thatdoes not erasure encode volume data across multiple mass storage devicesor such that performance exceeds a performance of a system that does noterasure encode volume data across multiple mass storage devices.

FIG. 7 is a high-level flowchart illustrating operations performed by ahead node in response to a request to store data in a data storage unit,according to some embodiments.

At 702, upon receiving a write request from a client device, wherein thewrite request is directed to a particular volume for which the head nodeis functioning as a primary head node, the head node writes dataincluded with the write request to the log of the head node and updatesthe index of the head node to include an entry for the volume data and apointer indicating where the volume data is stored.

At 704, the primary head node causes the data included with the writerequest to be replicated to the secondary head node. The secondary headnode then stores the data in a log of the secondary head node andupdates an index of a storage of the secondary head node to include anentry for the volume data and a pointer indicating where the volume datais stored. The secondary head node may then send an acknowledgement tothe primary head node indicating that the data has been replicated inthe secondary head node's storage. In some embodiments, the primary headnode then issues an acknowledgement to the client device indicating thatthe requested write has been persisted in the data storage system. Insome embodiments, replication between head nodes could be primary andsecondary e.g. master/slave replication. In some embodiments, otherreplication techniques such as a Paxos protocol, other consensusprotocol, etc. may be used to replicate data between head nodes.

At 706, the primary head node determines if the log data of the primaryhead node exceeds a threshold that would trigger the log data or asegment of the primary head node's log data to be flushed to extentsthat include columns of mass storage devices of data storage sleds of adata storage unit that includes the head node. In some embodiments, athreshold to trigger data to be flushed may include: an amount of datastored in the log or in a segment of the log, an amount of time that haselapsed since the data was last accessed or altered, a frequency atwhich the data is accessed or altered, or other suitable thresholds. Insome embodiments, data flushed from a log of a head node may onlyinclude a portion of the data written to the log of the head node or asegment of the log of the head node. For example, older data stored in alog of a head node may be flushed while more recently written data mayremain in the log of the head node. In some embodiments, a frequency offlush operations from a log of a head node may be throttled based on avariety of factors, such as a fill rate of the log of the head node orbased on an amount of write requests being received by the head node orbeing received for a particular volume serviced by the head node.

In response to determining the threshold has not been met, the primaryhead node continues to write data to the log and reverts to 702.

At 708, in response to determining that the threshold has been met orexceeded, the primary head node causes data stored in the log of theprimary head node or a segment of the log of the primary head node to beflushed to columns of mass storage devices in different ones of aplurality of data storage sleds of the data storage unit.

At 710, the primary head node updates the log of the primary head nodeto include a pointer for the volume data indicating that the flushedvolume data is now stored in particular columns of mass storage devicesor an extent that includes multiple columns of mass storage devices.

At 712, the primary head node causes the secondary head node to updatean index of the secondary head node to indicate the new location of thevolume data. The secondary head node also releases the log space in thesecondary head node that previously stored the replicated volume data.

At 714, the head node acting as primary head node also releases space inthe primary head node's log. In some embodiments, a garbage collectionmechanism may cause log space to be released based on inspecting anindex of a storage of a head node. In some embodiments, releasing logstorage space may be performed concurrently with flushing log data ormay be performed at some time subsequent to flushing log data.

FIG. 8A is a high-level flowchart illustrating operations performed by ahead node in response to a failed mass storage device in a data storagesled of a data storage unit, according to some embodiments.

At 802, a head node or a sled controller detects a failed mass storagedevice in a particular data storage sled. For example, a data storagesled may include multiple mass storage devices, such as solid statestorage drives, and one of the mass storage devices may fail. In someembodiments, a data storage sled may include disk drives and one of thedisk drives may fail. In some embodiments, a data storage sled mayinclude other types of mass storage devices.

At 804, a head node acting as a primary head node for a volume withextents that include one or more columns on the failed mass storagedevice or a local control plane for the data storage unit causes theextents that include columns on the failed mass storage device to bereplicated to other extents that include columns on other mass storagedevices in other sleds of the data storage unit. For example, in a 4+2erasure coding scheme data from any one lost mass storage drive can berecreated based on data stored on the other mass storage devices thatmake up an extent. Thus, data previously stored on the failed massstorage device can be recreated and replicated to data storage sledsthat do not include a failed mass storage device.

At 806, indexes of a primary head node and a secondary head node thatare designated for each volume that included an extent in the failedmass storage device are updated to indicate the new locations of thedata for the volumes.

In some embodiments, a data storage system may continue to operate adata storage sled that includes a failed mass storage device, such asthe failed mass storage device at 808. In some embodiments, step 806 maybe omitted and all extents stored on mass storage devices in the datastorage sled that includes the failed mass storage device may bereplicated to other data storage sleds. Because the extents that includecolumns on the failed mass storage device have been replicated to datastorage sleds that do not include failed mass storage devices, thedurability of the data previously stored on the failed mass storagedevice has been recovered to the original level of durability. Forexample in a RAID configuration of six segments, the number of segmentsis returned to six by replicating the data from the failed mass storagedevice to other mass storage devices in the data storage unit.

FIG. 8B is a high-level flowchart illustrating operations performed by ahead node in response to a failed mass storage device in a data storagesled of a data storage unit, according to some embodiments.

In some embodiments, a data storage system may tolerate one or morefailed mass storage devices in a particular sled before the mass storagedevices are replaced. For example, at 852 one or more additional failedmass storage devices are detected in a data storage sled. In someembodiments the additional failed mass storage devices may be in thesame data storage sled as the failed mass storage device described inFIG. 8A or may be in a different data storage sled of the data storageunit.

At 854, data from other non-failed mass storage devices each in a datastorage sled that includes a failed mass storage device is copied toother mass storage devices in other data storage sleds of the datastorage unit. In some embodiments, only data from non-failed massstorage devices that are included in a data storage sled that is to berepaired may be copied. In some embodiments, copying the data from thenon-failed mass storage devices may include recreating the data from aset of columns stored on remaining non-failed mass storage devices andthen erasure encoding the data across another set of columns of massstorage devices of a replacement extent. For example, in a 4+2 erasureencoding scheme, data of an extent may be recreated from any four of thesix columns of the extent. After being recreated, the data may beerasure encoded across another set of 4+2 columns of a replacementextent.

At 856, indexes of a primary head node and a secondary head node thatare designated for each volume that included an extent in the affectedmass storage devices are updated to indicate the new locations of thedata for the volumes that has been copied to other mass storage devicesin the data storage unit.

At 858, the data storage sled(s) that includes the failed mass storagedevice is at least partially removed from the data storage unit and thefailed mass storage device is replaced. Because data previously storedon the non-failed mass storage devices of the data storage sled beingremoved has been copied to other mass storage devices of the datastorage unit, the data remains available even while the data storagesled is at least partially removed from the data storage unit.

At 860, the data storage sled with the replaced mass storage device isre-installed in the data storage unit. At 862 mass storage devices ofthe replaced data storage sled are made available for allocation ofcolumns on the mass storage devices of the data storage sled. In someembodiments, data storage space of the non-failed mass storage devicesof the data storage sled may be released and made available to storedata for newly allocated extents. In some embodiments, the non-failedmass storage devices may still store volume data that has been copied toother mass storage devices in the data storage unit. In someembodiments, the indexes of the respective head nodes may be updated toindicate volume data that is still stored on the non-failed mass storagedevices.

Multi-Tier Control Plane

In some embodiments, a data storage system may include multiple datastorage units. Management of the data storage system may be performed bya multi-tiered control plane. For example, in some embodiments a zonalcontrol plane may determine which data storage units new volumes are tobe allocated to and may perform migration of volumes between datastorage units to balance loads. Also, in some embodiments, a localcontrol plane of a data storage unit may determine which head nodes ofthe data storage unit are to be assigned to a particular volume orvolume partition as a primary head node and a secondary head node. Also,a local control plane may manage allocation of extents within a datastorage unit via a “sandbox” technique and may perform fail overoperations in response to a failure of a head node, a mass storagedevice, or a data storage sled. In some embodiments, a data storage unitmay operate autonomously from a zonal control plane subsequent to avolume being assigned to the data storage unit. Because data storageunits may operate autonomous from a zonal control plane, a failure of azonal control plane may not impact a data storage unit's ability torespond to read and write requests or perform fail-over operations inresponse to a failure of a head node or a mass storage device. Also,because a local control plane of a data storage unit only affects asingle data storage unit, a failure of a local control plane may have ablast radius that is limited to a single data storage unit. Furthermore,a data storage unit may implement a local control plane on one or morehead nodes of a data storage unit and implement a lease protocol toallow for fail over of the local control plane from one head node toanother head node in response to a failure of a head node implementingthe local control plane. In some embodiments, a local control plane mayutilize a distributed value store that is distributed across theplurality of head nodes of the data storage unit. Thus, when aparticular head node implementing a local control plane fails, anotherhead node taking over implementation of the local control plane mayutilize the distributed value store without values in the value storebeing lost due to the failure of the head node previously implementingthe local control plane.

FIG. 9A is a block diagram illustrating a process for creating a volumeinvolving a zonal control plane, a local control plane, and head nodesof a data storage system, according to some embodiments. Data storagesystem 900 includes one or more computing devices that implement zonalcontrol plane 904 and also includes data storage units 906, 928, and930. Data storage units 906, 928, and 930 may be the same as any of thedata storage units described in FIGS. 1-8. Data storage unit 906includes head nodes 908, 912, and 914 and data storage sleds 916. Alocal control plane 910 is implemented on head node 908. Data storageunit 928 includes head nodes 918, 920, and 924 and data storage sleds926. A local control plane 922 for data storage unit 928 is implementedon head node 920. Data storage unit 930 includes head nodes 932, 934,and 936 and sleds 940. A local control plane 938 for data storage unit930 is implemented on head node 936. As can be seen a local controlplane for a data storage unit can be implemented on any one or more headnodes of a plurality of head nodes of a data storage unit. In someembodiments, a local control plane may be logically separated from adata plane of a head node, for example a local control plane may belocated in a separate container. In some embodiments, each head node ofa data storage unit may include logically isolated program instructionsfor implementing a local control plane and a portion of a distributedvalue store distributed across logically isolated portions of respectiveones of the head nodes. In such embodiments, a given head node holding alease for implementing the local control plane may implement the localcontrol plane using the program instructions stored in the given headnode. Upon failure of the given head node, another given head node mayassume the lease for implementing the local control plane and mayimplement the local control plane using the program instructions forimplementing the local control plane stored in the other given headnode. For example, the given head node and the other given head node mayboth store program instructions for implementing the local control planeand a single one of the given head node or the other given head node mayimplement the local control plane at a given time.

Client device(s) 902 may be part of a separate network that is separatefrom data storage system 900, such as a customer network, or may beclient devices within a provider network that utilizes data storagesystem 900. Client device(s) 902 send volume request A 942 and volumerequest B 944 to zonal control plane 904 to request volumes of datastorage system 900 be allocated to the client devices. In response,zonal control plane 904 issues a volume creation instruction A 946 todata storage unit 906 and a volume creation instruction B 948 to datastorage unit 928. In some embodiments, volume creation instructions froma zonal control plane may be processed by a local control plane of adata storage unit. For example, local control plane 910 of data storageunit 906 processes volume creation instruction A 946 and local controlplane 922 of data storage unit 928 processes volume creation instructionB 948. In some embodiments, a zonal control plane may receiveaccumulated performance and usage metrics from data storage units andassign volumes, based on the accumulated performance and usage metrics.For example, a zonal control plane may attempt to balance loads betweendata storage units by selecting to assign new volumes to data storageunits that have accumulated performance and usage metrics that indicateless load than other data storage units.

In order to process a volume creation instruction, a local control planemay assign a head node of a data storage unit to function as a primaryhead node for a volume and may assign another head node of a datastorage unit to function as a secondary head node for the volume. Forexample, local control plane 910 assigns head node 912 as a primary headnode for the newly created volume via assignment 950 and assigns headnode 914 as secondary head node for the volume via assignment 952. Also,local control plane 922 of data storage unit 928 assigns head node 918as a primary head node for a newly created volume via assignment 956 andassigns head node 924 as a secondary head node for the volume viaassignment 958. As can be seen, any one of the head nodes of a datastorage unit may be selected to function as a primary or secondary headnode for a given volume or volume partition. Also, a local control planeof a data storage unit may collect performance information from headnodes and select primary and secondary head nodes for a given volumebased on a current loading of head nodes in a data storage unit. In someembodiments, a local control plane may attempt to balance loads betweenhead nodes when assigning primary and secondary head nodes for a givenvolume.

FIG. 9B is a block diagram illustrating head nodes of a data storageunit servicing read and write requests independent of a zonal controlplane of a data storage system, according to some embodiments. Datastorage system 900 illustrated in FIG. 9B is the same data storagesystem 900 as illustrated in FIG. 9A and shows read and write requestsbeing sent to head nodes after primary and secondary head nodes for thenewly created volumes have been assigned. As can be seen, read and writerequests may be serviced by data storage units of a data storage systemindependent of a zonal control plane. Also, in some embodiments, readand write requests may be serviced independent of a local control plane.For example read request 968 and write request 960 are directed directlyto head node 912 that functions as a primary head node for the newlycreated volume. Also, primary head node sends read data 972 and writeacknowledgement 964 to client device(s) 902 without passing the readdata 972 or the write acknowledgment 964 through zonal control plane 904or local control plane 910. In some embodiments, each head node may beassigned at least one public network address, such as a public IPaddress. When a head node is assigned to function as a primary head nodefor a volume of a client, the primary head node's public IP address maybe communicated to the client device and the client device's address maybe communicated to the head node functioning as primary head node. Thus,subsequent communications between the head node and the client devicemay be directly routed between the exchanged address information. Insome embodiments, read and write requests may be directed to both aprimary and secondary head node. For example, if a client does notreceive a response from a primary head node, the client may direct aread or write request to a secondary head node. In some embodiments,this may trigger a secondary head node to attempt to assume the role ofprimary head node.

FIG. 10A is a block diagram of a head node, according to someembodiments. Head node 1000 may be any of the head nodes described inFIG. 1-9 or 11-19. Head node 1000 includes a data control plane 1002,storage 1010, local control plane 1004, and monitoring module 1016. Adata control plane of a head node, such as data control plane 1002, mayservice read and write requests directed to the head node. For example,a data control plane may store one or more public IP addresses of thehead node and provide the public IP addresses of the head node to clientdevices to allow the client devices to communicate with the head node. Astorage of a head node, such as storage 1010, may include a log, such aslog 1012, and an index, such as index 1014. Log 1012 and index 1014 maybe similar to log 408 and index 406 as described in regard to FIGS. 4Aand 4B and may store pointers for volume data indicating where thevolume data is stored. In some embodiments, a data control plane, suchas data control plane 1002, may consult an index, such as index 1014, inorder to service read and write requests directed at a particular volumefor which the head node is functioning as a primary head node. In someembodiments, an index, such as index 1014 may indicate whether a portionof volume data for a volume is stored in a log of the head node, such aslog 1012, or is stored in an extent across multiple data storage sleds,such as mass storage devices 1022 of data storage sled 1018 illustratedin FIG. 10B that also includes sled controller 1020. In addition, a headnode may include program instructions for implementing a local controlplane that are logically isolated from the data control plane of thehead node.

In some embodiments, a local control plane includes an extent allocationservice, such as extent allocation service 1006, and a distributed valuestore, such as value store 1008. An extent allocation service mayprovide “sandbox” recommendations to head nodes of a data storage unitthat include sets of columns from which the head nodes may select newextents. A value store may store extent allocation information and mayalso store head node assignment information. In some embodiments, alocal control plane may provide sequence numbers to newly assignedprimary head nodes. In some embodiments, a distributed value store, suchas value store 1008, may be implemented over all or a portion of thehead nodes of a data storage unit. This may provide fault tolerance suchthat if any one or more of the head nodes fail, the remaining head nodesmay include data from the distributed data store, such that data fromthe distributed data store is not lost due to the failure of the one ormore head nodes.

In some embodiments, a head node includes a monitoring module, such asmonitoring module 1016. Monitoring module may collect performance and/orusage metrics for the head node. A head node, such as head node 1000 mayprovide performance and/or usage metrics to a local control plane, suchas local control plane 1004, or may provide performance and/or usagemetrics to a zonal control plane.

FIG. 11 is a high-level flowchart illustrating a process of creating avolume in a data storage system, according to some embodiments.

At 1102, a local control plane of a data storage unit of a data storagesystem receives a volume assignment from a zonal control plane of thedata storage system.

At 1104, the local control plane assigns a first head node of the datastorage unit to function as a primary head node for the newly created ornewly assigned volume. At 1106, the local control plane assigns a secondhead node of the data storage unit to function as a secondary head nodefor the newly created or newly assigned volume. Note that in someembodiments, a zonal control plane may move volume between data storageunits of a data storage system. Thus the newly assigned volume may be anexisting volume being moved from another data storage unit of the datastorage system. Also, a local control plane of a data storage unit mayselect head nodes to function as primary and secondary head nodes fromany of the head nodes of the data storage unit. However, a head nodefunctioning as a primary head node may not function as a secondary headnode for the same volume. But, a given head node may function as aprimary head node for more than one volume and may also function as asecondary head node for one or more other volumes.

At 1108 the primary head node for the volume services read and writerequests directed at the volume. In some embodiments, a head nodefunctioning as a primary head node may service read and write requestsindependent of a zonal control plane and/or independent of a localcontrol plane of a data storage unit.

FIG. 12A is a high-level flowchart illustrating a local control plane ofa data storage unit providing storage recommendations to a head node ofthe data storage unit for locations to store data in data storage sledsof the data storage unit for a given volume, according to someembodiments.

At 1202, a local control plane of a data storage unit allocates a“sandbox” to a particular volume serviced by a primary head nodefunctioning as primary head node for the particular volume. The sandboxmay include a set of columns of mass storage devices from which the headnode is recommended to select extents for the particular volume. In someembodiments, the sandbox may include extents that already includecorresponding columns in multiple mass storage devices and the head nodemay be recommended to select extents for the particular volume from theextents included in the sandbox recommendation.

At 1204, the local control plane collects performance metrics from datastorage sleds and/or head nodes in the data storage unit.

At 1206, the local control plane issues “sandbox’ updates to the primaryhead node functioning as a primary head node for the particular volume.The sandbox updates may be based on the collected performance metricscollected at 1204. A local control plane may allocate sandboxrecommendations and update sandbox recommendations to avoid heatcollisions wherein multiple head nodes are attempting to access the samedata storage sleds at the same time. In some embodiments, a sandboxrecommendation may be a loose constraint and a head node functioning asa primary head node may select columns or extents that are not includedin a sandbox recommendation. It should also be noted that sandboxrecommendation and performance and/or usage metrics collection may beperformed outside of the I/O path. Thus, if there is a failure orcorruption of the local control plane, reads and writes may continue tobe processed by non-affected head nodes of a data storage unit. Also, asandbox allocated to a particular volume may remain with the particularvolume during a failover of head nodes. For example, if a primary headnode for a particular volume fails, the sandbox allocated for theparticular volume may move with the particular volume that will now beserviced by a former secondary head node. Subsequent to a head nodefailover, sandbox updates, such as the sandbox updates described at1206, may be issued from the local control plane to the new primary headnode for the volume.

FIG. 12B is a high-level flowchart illustrating a head node of a datastorage unit storing data in data storage sleds of the data storageunit, according to some embodiments.

At 1252, a primary head node determines a segment of data to be flushedto mass storage devices in data storage sleds of a data storage unit.For example, exceeding one or more thresholds, such as an amount of datastored in a log, an age of data stored in a log, or an infrequency atwhich the data is accessed in a log, may trigger a primary head node toflush data to data storage sleds.

At 1254, a primary head node may determine if there is available spacein a sandbox allocated to a volume serviced by the primary head node. At1256, in response to determining there is sufficient space in thesandbox, the primary head node flushes the data to extents that includecolumns in the allocated sandbox allocated for the volume. At 1258, inresponse to determining there is insufficient space in the sandbox or inresponse to determining a placement in the sandbox will violate aplacement restriction, such as a durability level, the primary head nodeselects extents outside of the sand box.

FIG. 13 is a high-level flowchart illustrating head nodes of a datastorage unit performing a fail over operation in response to a failureof or loss of communication with one of the head nodes of the datastorage unit, according to some embodiments.

At 1302 communication with a primary head node is lost or the primaryhead node fails. In some embodiments, a client device may lose contactwith a primary head node and the client device may contact the secondaryhead node. This may trigger the secondary head node to attempt to takeover as primary head node.

At 1304, in response to the secondary head node attempting to take overas primary head node, the local control plane issues a new sequencenumber to the secondary head node. The new sequence number may begreater than a sequence number previously issued to the previous primaryhead node. The new sequence number may be used by the secondary headnode to gain write access to extents that were previously reserved forwrite access only by the previous primary head node.

At 1306, the secondary head node assumes the role of primary head nodeand begins to service writes directed to the volume. In someembodiments, the secondary head node may assume the role of primary headnode by presenting the new sequence number received from the localcontrol plane to sled controllers of the data storage system andreceiving, from the sled controllers, credentials for writing to columnsthat store data of the volume.

At 1308, the local control plane designates another head node of thedata storage unit to function as a secondary head node for the volume orvolume partition. Note that the previous secondary head node has assumedthe role of primary head node, such that the volume is without asecondary head node causing the local control plane to designate a newsecondary head node.

At 1310, the new primary head node (previous secondary head node)replicates log and index data for the volume to the newly designatedsecondary head node. In some embodiments, replicating log and index datamay include replicating index data for the volume including pointers forvolume data stored in data storage sleds of a data storage unit andvolume data stored in the log of the new primary head node (previoussecondary head node) that has not yet been flushed to the data storagesleds.

FIG. 14 is a block diagram illustrating performance and/or usage metricsbeing collected and accumulated in a data storage unit, according tosome embodiments.

Data storage system 1400 may be the same as data storage system 900illustrated in FIG. 9. Data storage system includes zonal control plane1404 and data storage units 1406, 1428, and 1430. In some embodiments,data storage sleds and head nodes of a data storage unit may reportperformance and usage metrics to a local control plane for the datastorage unit. For example, head nodes 1408, 1412, and 1414 of datastorage unit 1406 report performance and usage metrics to local controlplane 1410 of data storage unit 1406. Also, a sled controller of each ofdata storage sleds 1416 may report performance and usage metrics tolocal control plane 1410. In a similar manner, data storage sleds 1426and head nodes 1418, 1420, and 1424 of data storage unit 1428 may reportperformance and usage metrics to local control plane 1422 of datastorage unit 1428. Likewise, data storage sleds 1440 and head nodes1432, 1434, and 1436 of data storage unit 1430 may report performanceand usage metrics to local control plane 1438. In some embodiments, eachlocal control plane of a data storage unit may in turn reportaccumulated performance and usage metrics to a zonal control plane forthe data storage system. For example, local control planes 1410, 1422,and 1438 report performance and usage metrics to zonal control plane1404. In some embodiments local control planes may use performance andusage metrics to balance loads between head nodes and to update sandboxrecommendations that indicate recommended data storage sleds from whichhead nodes should select extents for a given volume. Also, a zonalcontrol plane may use cumulative performance and usage metrics tobalance volume assignments and/or move volumes between data storageunits. In some embodiments, performance and usage metrics may be used bya local control plane to balance loads within a given data storage unitand accumulated performance and usage metrics may be used by a zonalcontrol plane to balance loads between data storage units.

Input/Output Fencing of Mass Storage Devices from Unauthorized HeadNodes

In some embodiments, a sled controller of a data storage sled mayimplement a fencing protocol that prevents unauthorized head nodes fromwriting data to columns of mass storage devices located in a datastorage sled along with the sled controller. In some embodiments, a sledcontroller may issue credentials or tokens to head nodes for accessingcolumns allocated to a particular volume serviced by the respective headnodes. The sled controller may only issue a new token to a head node ifa column associated with the credential or token is not currentlyreserved or if a head node seeking to access the column presents asequence number greater than a sequence number stored for the columnthat indicates a sequence number of a previous head node that requestedto access the column. For example, a newly designated primary head nodefor a given volume may receive from a local or zonal control plane asequence number for the given volume that is greater than a previouslyissued sequence number for the given volume. The newly designatedprimary head node may then present the new sequence number to sledcontrollers of data storage sleds that include columns allocated for thevolume. The sequence number of the newly designated primary head nodemay be greater than a sequence number stored in the columns thatcorresponded to a sequence number of a previous primary head node thataccessed the columns. Upon determining that the newly designated primaryhead node has presented a sequence number greater than a stored sequencenumber, the sled controllers may issue a new token to the newlydesignated primary head node for accessing the columns.

For example, FIG. 15 illustrates interactions between a local controlplane, head nodes, and data storage sleds of a data storage unit inrelation to writing data to mass storage devices of a data storage sledof the data storage unit, according to some embodiments. Variousinteractions are illustrated between local control plane 1502 of datastorage unit, head nodes 1504 and 1506 of the data storage unit and sledcontrollers 1508 of the data storage unit. Any of the data storage unitsdescribed herein may include a local control plane, head nodes and sledcontrollers of data storage sleds that function as described in FIG. 15.

Phases 1, 2, and 3 are illustrated to show interactions that take placeat different phases of operation of a data storage system. For example,phase 1 may be a normal phase in which a head node is assuming the roleof primary head node for a volume or volume partition and functioning asthe primary head node for the volume or volume partition. Phase 2 mayrepresent a failover phase in which a secondary head node is assumingthe role of primary head node for the volume, and phase 3 may representa new normal phase wherein a newly designated primary head node isfunctioning as a primary head node for the volume.

At phase 1, local control plane 1502 assigns (1510) head node 1504 to bea primary head node for a volume and assigns (1512) head node 1506 to bea secondary head node for the volume. Assignment 1510 may include a newsequence number that is a monotonically increasing number that isgreater than all sequence numbers previously issued by the local controlplane 1502. At phase 1, in order to reserve columns of mass storagedevices in different ones of multiple data storage sleds of a datastorage unit, head node 1504 presents (1514) the new sequence number tosled controllers 1508 and reserves (1514) columns on mass storagedevices located in data storage sleds that include the sled controllers1508. At 1516, the sled controllers issue credentials or tokens to headnode 1504 indicating that the columns are reserved for the volume andthat head node 1504 is functioning as primary head node for the volume.At 1518, head node 1504 then issues a write request to sled controllers1508 and includes along with the write requests the tokens orcredentials issued by the sled controllers. The sled controllers verifythat the credentials or tokens included with the write request arevalid, perform the requested write, and at 1520 issue a writeacknowledgement to head node 1504. Also the sled controllers store thesequence number and volume ID or volume partition ID in each columnalong with the data included with the write request.

During phase 2 or the fail over phase, communication is lost with headnode 1504 at 1522. In some embodiments, loss of communication with aprimary head node may be triggered by a client device failing to be ableto reach the primary head node and instead contacting the secondary headnode. In such embodiments, the secondary head node may attempt to takeover as primary head node (not illustrated in FIG. 15). In someembodiments, a local control plane may determine that a primary headnode has been lost. In response to determining that a primary head nodehas failed or there is a loss of communication with a primary head node,at 1524, local control plane 1502 promotes head node 1506 to primaryhead node for the volume and issues a new sequence number to head node1506. Head node 1506 then, at 1526, presents the new sequence numberissued to head node 1506 to sled controllers 1508 and requests access tothe columns that store data for the volume for which head node 1506 isnow the primary head node. The new sequence number issued to head node1506 is greater than the sequence number issued to head node 1504 at1510. At 1528, the sled controllers issue a new token or credential tohead node 1506 that supersedes the token or credential issued to headnode 1504 at 1516.

During phase 3, head node 1506 functions as a primary head node for thevolume. At 1530 head node 1506 includes with subsequent write requeststhe tokens issued from the sled controllers at 1528. At 1532 sledcontrollers acknowledge subsequent writes from head node 1506. Also, at1534 head node 1504 that has lost communication with control plane 1502and/or head node 1506 attempts to perform a write to columns assigned tothe volume. However, subsequent to the failover, head node 1504 is nolonger the primary head node for the volume and head node 1506 isfunctioning as primary head node for the volume. Thus, head node 1506has exclusive access to columns of mass storage devices of extentsallocated to the volume. Thus, at 1534 when head node 1504 attempts toaccess the columns sled controllers 1508 decline (1536) to perform thewrite. In addition, at 1536 the head node 1504 may read the volume IDand new sequence number stored in the columns assigned to the volume.The columns may store the new sequence number issued to head node 1506during the failover. Upon determining that a new sequence number hasbeen stored that supersedes the sequence number last issued to head node1504, head node 1504 may determine that it is no longer primary headnode for the volume and may assume a role of secondary head node for thevolume.

Note that each column stores a volume or volume partition ID for avolume for which the column is allocated along with a most recentsequence number. The volume ID and sequence number may be saved inpersistent memory of the column. Also, a sled controller may storevolume ID and sequence number information in a volatile memory of thesled controller. However, when a sled controller is reset, e.g. losespower, the volume and sequence number stored in the sled controller maybe lost. However, volume and sequence number information stored incolumns of mass storage devices may be persisted. This avoidscomplications that may arise if mass storage devices are moved betweendata storage sleds. For example, if a mass storage device is movedwithin a data storage sled or amongst data storage sleds, sledcontroller volume ID and sequence number information may becomeinaccurate. However, because volume ID and sequence number informationis lost from a sled controller whenever power is lost to the sledcontroller, the sled controller may be reset when a sled is removed froma data storage unit to access mass storage devices in the data storagesled avoiding such complications. Thus, subsequent to a reboot of a sledcontroller, head nodes serving as primary head nodes for volumes thathave columns allocated on a sled that includes the sled controller mayneed to reclaim the columns. For example the head nodes may presentrespective sequence numbers issued to the head nodes and the sledcontrollers may issue new credentials or tokens to the head nodes if thesequence numbers presented have not be superseded, e.g. the sequencenumbers stored in the columns are not greater than the sequence numbersbeing presented by the head nodes.

FIG. 16 is a high-level flowchart of a head node of a data storage unitflushing data stored in a storage of the head node to a data storagesled of the data storage unit, according to some embodiments.

At 1602, a head node functioning as a primary head node for a volumereceives a write request. At 1604, the head node writes data includedwith the write request to a storage of the head node, such as a log ofthe head node.

At 1606, in response to determining data stored in the storage of thehead node exceeds a threshold, the head node requests sled controllersof multiple data storage sleds cause portions of the data stored in thestorage of the head node be stored in multiple portions of differentmass storage devices in different ones of the data storage sleds of thedata storage unit. Requesting the sled controllers to store the data mayfurther include presenting credentials (1608), such as credentialsdescribed in FIG. 15, to each of the sled controllers.

FIG. 17 is a high-level flowchart of a sled controller of a data storagesled processing a write request, according to some embodiments.

At 1702, a sled controller receives a credential from a head node alongwith a write request. At 1704 and 1706, the sled controller determinesif the credential received at 1702 is a currently valid credential for acolumn of a mass storage device in a data storage sled that includes thesled controller. A sled controller may compare a sequence number and/orvolume ID included in the credential with a sequence number and/orvolume ID saved in the column for which access is requested. If thesequence number and/or volume ID included in the credential match thesequence number and/or volume ID stored in the column the sledcontroller may determine that the credential is valid. In someembodiments, a sled controller may store information that correspondswith a token or credential, such as a token number. If the informationthat corresponds with the token stored by the sled controller matchesinformation included in the token, the sled controller may determine thecredential or token is a valid credential. If a sequence number includedin the credential or token is inferior to a sequence number stored inthe column, the sled controller may determine that the credential ortoken is invalid. In some embodiments, a head node may not currentlyhave credentials for a particular column and may present a sequencenumber that is greater than a stored sequence number stored for thecolumn and the sled controller may issue credentials that supersede allpreviously issued credentials for the column, such as a new token thatsupersedes all tokens previously issued for the column.

At 1712, in response to determining at 1706 that the credential includedwith the write request is an invalid credential, the sled controllerdoes not perform the requested write and returns a message to the headnode indicating that the credential is invalid.

At 1708, in response to determining the credential is valid, the sledcontroller performs the requested write to the requested column of amass storage device in the data storage sled along with the sledcontroller. At 1710 the sled controller acknowledges the write has beenperformed to the head node.

Data Storage Unit Design with Redundant Networks and Redundant Power

In some embodiments, a data storage unit may include redundant networkand redundant power supplies and power distribution systems. Suchredundant systems may reduce probabilities of failure thus allowing, forexample, a single rack to store all parts of a volume while stillmeeting customer requirements for reliability and data durability.However, in some embodiments, a volume or volume partition may be storedin more than one data storage unit.

FIGS. 18A-D illustrate a data storage unit with redundant network pathswithin the data storage unit, according to some embodiments. Datastorage unit 1850 illustrated in FIGS. 18A-D may be the same as datastorage unit 100 illustrated in FIG. 1, or any other data storage unitdescribed herein. FIGS. 18A-D further illustrate communication pathsbetween network switches 1802 and 1804, head nodes 1806-1808, and datastorage sleds 1834-1844. As can be seen, at least two redundantnetworks, including internal network 1852 and internal network 1854, areimplemented within data storage unit 1850. Note that paths betweencomponents of data storage unit 1850 are illustrated on either side ofdata storage unit 1850 for clarity, but in practice paths betweencomponents of a data storage unit may be within the data storage unitover wires, cables, busways, etc. of the data storage unit.

In FIG. 18A redundant communication paths are established between headnode 1806 and network 1828 via network switches 1802 and 1804. In someembodiments, a head node, such as head node 1806, may be assignedredundant network addresses routable from devices external to datastorage unit 1850, such as public IP addresses, and may be reachable viaeither one of the network address using either one of network switches1802 and 1804.

FIG. 18B illustrates redundant network paths between head nodes. Forexample head node 1806 may reach head node 1808 via internal network1852 or internal network 1854, wherein internal network 1852 is vianetwork switch 1802 and internal network 1854 is via network switch1804. Note that there is a single network hop between head node 1806 andhead node 1808 via network switch 1802 or network switch 1804. In someembodiments, a data storage unit may have a single network hop betweenhead nodes and data storage sleds so that input/output operations do notrequire multiple network hops to retrieve or write data, thus improvingIOPS performance and latency.

FIG. 18C illustrates redundant network paths between head nodes and datastorage sleds. For example, head node 1806 may reach any of data storagesleds 1834-1844 via sled controllers 1812-1822. Each sled controller mayinclude two network ports that each are connected to different ones ofinternal networks 1852 or 1854 via either one or network switches 1802or 1804. In some embodiments, each head node may be assigned at leasttwo private network addresses and each sled controller may be assignedat least two private network addresses. The private network addressesassigned to the head nodes and sled controllers of the data storagesleds may enable the head nodes and sled controllers to communicate witheach other via either one of internal networks 1852 or 1854. FIG. 18Dillustrates a head node sending a response communication to a clientdevice via either one of internal networks 1852 or 1854.

In some embodiments, a data storage unit may be configured to acceptmore or less head nodes in a rack of the data storage unit or to acceptmore or less data storage sleds in the rack of the data storage unit.Thus, compute capacity and data storage capacity of a data storage unitmay be adjusted by varying a quantity of head nodes and/or data storagesleds that are included in the data storage unit.

FIGS. 19A-C illustrate a data storage unit configured to allow scalingof storage capacity and processing capacity, according to someembodiments. For example, data storage unit 1902 is shown in arrangement1900 in FIG. 19A and in arrangement 1920 in FIG. 19B. In arrangement1920 data storage unit 1902 includes more data storage sleds than inarrangement 1900. Also, in arrangement 1940, data storage unit 1902includes more head nodes than in arrangement 1900. In some embodiments,a ratio of head nodes to data storage sleds may be adjusted to meetcustomer needs.

Example Computer System

FIG. 20 is a block diagram illustrating an example computer system,according to various embodiments. For example, computer system 2000 maybe configured to implement storage and/or head nodes of a data storageunit, storage and/or a sled controller of a data storage sled, otherdata stores, and/or a client, in different embodiments. Computer system2000 may be any of various types of devices, including, but not limitedto, a personal computer system, desktop computer, laptop or notebookcomputer, mainframe computer system, handheld computer, workstation,network computer, a consumer device, application server, storage device,telephone, mobile telephone, or in general any type of computing device.

Computer system 2000 includes one or more processors 2010 (any of whichmay include multiple cores, which may be single or multi-threaded)coupled to a system memory 2020 via an input/output (I/O) interface2030. Computer system 2000 further includes a network interface 2040coupled to I/O interface 2030. In various embodiments, computer system2000 may be a uniprocessor system including one processor 2010, or amultiprocessor system including several processors 2010 (e.g., two,four, eight, or another suitable number). Processors 2010 may be anysuitable processors capable of executing instructions. For example, invarious embodiments, processors 2010 may be general-purpose or embeddedprocessors implementing any of a variety of instruction setarchitectures (ISAs), such as the x86, PowerPC, SPARC, or MIPS ISAs, orany other suitable ISA. In multiprocessor systems, each of processors2010 may commonly, but not necessarily, implement the same ISA. Thecomputer system 2000 also includes one or more network communicationdevices (e.g., network interface 2040) for communicating with othersystems and/or components over a communications network (e.g. Internet,LAN, etc.).

In the illustrated embodiment, computer system 2000 also includes one ormore persistent storage devices 2060 and/or one or more I/O devices2080. In various embodiments, persistent storage devices 2060 maycorrespond to disk drives, tape drives, solid state memory, other massstorage devices, block-based storage devices, or any other persistentstorage device. Computer system 2000 (or a distributed application oroperating system operating thereon) may store instructions and/or datain persistent storage devices 2060, as desired, and may retrieve thestored instruction and/or data as needed. For example, in someembodiments, computer system 2000 may host a storage unit head node, andpersistent storage 2060 may include the SSDs that include extentsallocated to that head node.

Computer system 2000 includes one or more system memories 2020 that areconfigured to store instructions and data accessible by processor(s)2010. In various embodiments, system memories 2020 may be implementedusing any suitable memory technology, (e.g., one or more of cache,static random access memory (SRAM), DRAM, RDRAM, EDO RAM, DDR 10 RAM,synchronous dynamic RAM (SDRAM), Rambus RAM, EEPROM,non-volatile/Flash-type memory, or any other type of memory). Systemmemory 2020 may contain program instructions 2025 that are executable byprocessor(s) 2010 to implement the methods and techniques describedherein. In various embodiments, program instructions 2025 may be encodedin platform native binary, any interpreted language such as Java™byte-code, or in any other language such as C/C++, Java™, etc., or inany combination thereof. For example, in the illustrated embodiment,program instructions 2025 include program instructions executable toimplement the functionality of a storage node, in different embodiments.In some embodiments, program instructions 2025 may implement multipleseparate clients, nodes, and/or other components.

In some embodiments, program instructions 2025 may include instructionsexecutable to implement an operating system (not shown), which may beany of various operating systems, such as UNIX, LINUX, Solaris™, MacOS™,Windows™, etc. Any or all of program instructions 2025 may be providedas a computer program product, or software, that may include anon-transitory computer-readable storage medium having stored thereoninstructions, which may be used to program a computer system (or otherelectronic devices) to perform a process according to variousembodiments. A non-transitory computer-readable storage medium mayinclude any mechanism for storing information in a form (e.g., software,processing application) readable by a machine (e.g., a computer).Generally speaking, a non-transitory computer-accessible medium mayinclude computer-readable storage media or memory media such as magneticor optical media, e.g., disk or DVD/CD-ROM coupled to computer system2000 via I/O interface 2030. A non-transitory computer-readable storagemedium may also include any volatile or non-volatile media such as RAM(e.g. SDRAM, DDR SDRAM, RDRAM, SRAM, etc.), ROM, etc., that may beincluded in some embodiments of computer system 2000 as system memory2020 or another type of memory. In other embodiments, programinstructions may be communicated using optical, acoustical or other formof propagated signal (e.g., carrier waves, infrared signals, digitalsignals, etc.) conveyed via a communication medium such as a networkand/or a wireless link, such as may be implemented via network interface2040.

In some embodiments, system memory 2020 may include data store 2045,which may be configured as described herein. In general, system memory2020 (e.g., data store 2045 within system memory 2020), persistentstorage 2060, and/or remote storage 2070 may store data blocks, replicasof data blocks, metadata associated with data blocks and/or their state,configuration information, and/or any other information usable inimplementing the methods and techniques described herein.

In one embodiment, I/O interface 2030 may be configured to coordinateI/O traffic between processor 2010, system memory 2020 and anyperipheral devices in the system, including through network interface2040 or other peripheral interfaces. In some embodiments, I/O interface2030 may perform any necessary protocol, timing or other datatransformations to convert data signals from one component (e.g., systemmemory 2020) into a format suitable for use by another component (e.g.,processor 2010). In some embodiments, I/O interface 2030 may includesupport for devices attached through various types of peripheral buses,such as a variant of the Peripheral Component Interconnect (PCI) busstandard or the Universal Serial Bus (USB) standard, for example. Insome embodiments, the function of I/O interface 2030 may be split intotwo or more separate components, such as a north bridge and a southbridge, for example. Also, in some embodiments, some or all of thefunctionality of I/O interface 2030, such as an interface to systemmemory 2020, may be incorporated directly into processor 2010.

Network interface 2040 may be configured to allow data to be exchangedbetween computer system 2000 and other devices attached to a network,such as other computer systems 2090, for example. In addition, networkinterface 2040 may be configured to allow communication between computersystem 2000 and various I/O devices 2050 and/or remote storage 2070.Input/output devices 2050 may, in some embodiments, include one or moredisplay terminals, keyboards, keypads, touchpads, scanning devices,voice or optical recognition devices, or any other devices suitable forentering or retrieving data by one or more computer systems 2000.Multiple input/output devices 2050 may be present in computer system2000 or may be distributed on various nodes of a distributed system thatincludes computer system 2000. In some embodiments, similar input/outputdevices may be separate from computer system 2000 and may interact withone or more nodes of a distributed system that includes computer system2000 through a wired or wireless connection, such as over networkinterface 2040. Network interface 2040 may commonly support one or morewireless networking protocols (e.g., Wi-Fi/IEEE 802.11, or anotherwireless networking standard). However, in various embodiments, networkinterface 2040 may support communication via any suitable wired orwireless general data networks, such as other types of Ethernetnetworks, for example. Additionally, network interface 2040 may supportcommunication via telecommunications/telephony networks such as analogvoice networks or digital fiber communications networks, via storagearea networks such as Ethernet, Fibre Channel SANs, or via any othersuitable type of network and/or protocol. In various embodiments,computer system 2000 may include more, fewer, or different componentsthan those illustrated in FIG. 20 (e.g., displays, video cards, audiocards, peripheral devices, other network interfaces such as an ATMinterface, an Ethernet interface, a Frame Relay interface, etc.)

It is noted that any of the distributed system embodiments describedherein, or any of their components, may be implemented as one or morenetwork-based services. For example, a compute cluster within acomputing service may present computing and/or storage services and/orother types of services that employ the distributed computing systemsdescribed herein to clients as network-based services. In someembodiments, a network-based service may be implemented by a softwareand/or hardware system designed to support interoperablemachine-to-machine interaction over a network. A network-based servicemay have an interface described in a machine-processable format, such asthe Web Services Description Language (WSDL). Other systems may interactwith the network-based service in a manner prescribed by the descriptionof the network-based service's interface. For example, the network-basedservice may define various operations that other systems may invoke, andmay define a particular application programming interface (API) to whichother systems may be expected to conform when requesting the variousoperations. though

In various embodiments, a network-based service may be requested orinvoked through the use of a message that includes parameters and/ordata associated with the network-based services request. Such a messagemay be formatted according to a particular markup language such asExtensible Markup Language (XML), and/or may be encapsulated using aprotocol such as Simple Object Access Protocol (SOAP). To perform anetwork-based services request, a network-based services client mayassemble a message including the request and convey the message to anaddressable endpoint (e.g., a Uniform Resource Locator (URL))corresponding to the network-based service, using an Internet-basedapplication layer transfer protocol such as Hypertext Transfer Protocol(HTTP).

In some embodiments, network-based services may be implemented usingRepresentational State Transfer (“RESTful”) techniques rather thanmessage-based techniques. For example, a network-based serviceimplemented according to a RESTful technique may be invoked throughparameters included within an HTTP method such as PUT, GET, or DELETE,rather than encapsulated within a SOAP message.

Although the embodiments above have been described in considerabledetail, numerous variations and modifications may be made as wouldbecome apparent to those skilled in the art once the above disclosure isfully appreciated. It is intended that the following claims beinterpreted to embrace all such modifications and changes and,accordingly, the above description to be regarded in an illustrativerather than a restrictive sense.

What is claimed is:
 1. A data storage system comprising: a plurality ofhead nodes; and a plurality of data storage sleds, wherein for apartition of a volume to be stored in the data storage system, aparticular one of the head nodes is designated as a primary head nodefor the volume partition and another one of the head nodes is designatedas a secondary head node for the volume partition, wherein, based, atleast in part, on receiving a write request for the volume partition,the head node designated as the primary head node for the volumepartition is configured to: write data included with the write requestto a storage of the head node designated as the primary head node; andcause the data included with the write request to be replicated to theother head node designated as the secondary head node; wherein the headnode designated as the primary head node for the volume partition isfurther configured to cause respective parts of the data stored in thestorage of the head node to be erasure encoded and stored in a pluralityof respective mass storage devices.
 2. The data storage system of claim1, wherein the plurality of mass storage devices that store the erasureencoded respective parts of the data are in different ones of theplurality of data storage sleds of the data storage system.
 3. The datastorage system of claim 1, wherein at least a portion of the datawritten to the storage of the head node designated as the primary headnode is caused to be erasure encoded and stored in the plurality ofrespective mass storage devices based, at least in part, on: a quantityof data stored in the storage of the head node designated as the primaryhead node exceeding a threshold amount of stored data; or an amount oftime, since data stored in the storage of the head node designated asthe primary head node was stored, exceeding a threshold amount of time.4. The data storage system of claim 1, wherein the head node designatedas the primary head node is configured to release storage space of thehead node subsequent to performing said cause respective parts of thedata stored in the storage of the head node to be stored in theplurality of respective mass storage devices; and wherein the head nodedesignated as the primary head node is configured to cause storage spaceof the other head node designated as the secondary head node to bereleased subsequent to performing said cause respective parts of thedata stored in the storage of the head node to be stored in theplurality of respective mass storage devices.
 5. A data storage systemcomprising: a head node of the data storage system; wherein, based, atleast in part, on receiving a write request for a volume partition, thehead node acting as a primary head node of the data storage system forthe volume partition is configured to: write data included with thewrite request to a storage of the head node; and cause the data includedwith the write request to be replicated to another head node of the datastorage system designated as a secondary head node for the volumepartition; wherein the head node is further configured to causerespective parts of the data stored in the storage of the head node tobe stored in a plurality of respective mass storage devices in differentones of a plurality of data storage sleds of the data storage system. 6.The data storage system of claim 5, wherein the data is stored at aparticular level of data durability when stored in the storage of thehead node acting as primary head node and the other the other head nodedesignated as secondary head node; and wherein the data is stored atanother level of data durability when the respective parts of the dataare stored in the plurality of respective mass storage devices each indifferent ones of the plurality of data storage sleds of the datastorage system.
 7. The data storage system of claim 6, wherein the headnode designated as the primary head node is configured to cause the dataincluded in the write request to be replicated to one or more additionalhead nodes.
 8. The data storage system of claim 6, wherein the head nodedesignated as the primary head node is configured to adjust a number ofdata storage sleds across which the data stored in the storage of thehead node is caused to be stored.
 9. The data storage system of claim 5,wherein the storage of the head node and the storage of the other headnode are log-structured storages.
 10. The data storage system of claim9, wherein the storage of the head node comprises an index, wherein forthe volume partition, the index identifies a location within the log ofthe head node at which the data included in the write request is stored,wherein the head node is configured to update the index to identifyrespective portions of the plurality of respective mass storage deviceseach in different ones of the plurality of data storage sleds where therespective parts of the data are stored.
 11. The data storage system ofclaim 5, wherein the head node is configured to cause respective partsof the data stored in the storage of the head node to be stored in aplurality of respective mass storage devices asynchronously with thedata being stored in the storage of the head node and in a storage ofthe head node designated as the secondary head node.
 12. The datastorage system of claim 5, wherein to cause the respective parts of thedata stored in the in the storage of the head node to be stored in theplurality of respective mass storage devices, the head node is furtherconfigured to erasure encode the data stored in the storage of the headnode across the plurality of mass storage devices.
 13. The data storagesystem of claim 5, wherein the head node is configured to be a secondaryhead node for another volume partition stored in the data storagesystem.
 14. The data storage system of claim 13, wherein the head nodedesignated as a secondary head node for the other volume partition doesnot cause data of the other volume partition to be stored in massstorage devices of the data storage sleds; and wherein the head node,upon being designated as a primary head node for the other volumepartition is configured to cause respective parts data of the othervolume partition stored in the storage of the head node to be stored ina plurality of respective mass storage devices.
 15. A non-transitorycomputer readable medium storing program instructions for implementing adata storage system, wherein the program instructions when executed by aprocessor cause the data storage system to: receive a write request fora volume partition: write data included with the write request to astorage of a head node designated as primary head node for the volumepartition; and replicate the data included in the write request toanother head node of the data storage system designated as a secondaryhead node for the volume partition; erasure encode at least a portion ofthe data stored in the storage of the head node designated as theprimary head node; and store respective parts of the erasure encodeddata in a plurality of respective mass storage devices each in differentones of a plurality of data storage sleds of the data storage system.16. The non-transitory computer readable medium of claim 15, wherein theprogram instructions when executed cause the data storage system toasynchronously perform said: replicate the data to another head node ofthe data storage system designated as a secondary head node for thevolume partition; and erasure encode the at least a portion of the dataand store the respective parts of the erasure encoded data.
 17. Thenon-transitory computer readable medium of claim 16, wherein the programinstructions when executed cause the data storage system to: causerespective parts of the data stored in the storage of the head nodedesignated as the primary head node to be stored in a plurality ofrespective mass storage devices, based, at least in part, on: an amountof time since the data was written to the storage of the head nodedesignated as primary head node; or an amount of data stored in thestorage of the head node designated as the primary head node exceeding athreshold amount of data.
 18. The non-transitory computer readablemedium of claim 15, wherein the program instructions when executed causethe data storage system to: erasure encode the data stored in thestorage of the head node designated as the primary head node across theplurality of mass storage devices.
 19. The non-transitory computerreadable medium of claim 15, wherein the erasure coded data is storedacross at least six mass storage devices, wherein at least four of themass storage devices store portions of the data and at least two of themass storage devices store coded data derived from the data.
 20. Thenon-transitory computer readable medium of claim 15, wherein the programinstructions when executed cause the data storage system to: detect afailure of one or more of the mass storage devices in a particular oneof the data storage sleds; and copy data stored in remaining massstorage devices of the data storage sled to mass storage devices in oneor more other data storage sleds.